Well, first off, a lot depends on what you want out of a box... for that matter, do you need a "box" at all? Well anyway... If you're just talking a sequencer, there's actually a lot of good sequencers, from basic to waaaay complex. However, many sequencers don't lend themselves to much live manipulation. I mean you usually have mutes, maybe, but you usually can't do a lot to alter the actual sequence on the fly. The "multi-boxes" like the RS7K, the MC505, the EMU XL-7, the RM1x, the MPC2K, and so on also suffer from a lack of real-time tweakability to one degree or other, too. Anyhoos, I'm thinkin' you were referring to these sorts of multi-boxes when you asked yer question, so...
I like 'em for the convenience factor, you usually get a fair amount of MIDI control, a sequencer, and maybe some usable sounds... Beats lugging a lot of extra shit... Most of the time you'll probably want another sound source or two, but for the stuff I do it certainly cuts down on how much kit I need to carry.
Now, the "best" box? Well, as I answered to Influx, there ain't no such thing... All of them have shortcomings, given how and what I like to play. If I had to pick one single box from those that are available for live use, it'd probably still be the MC505 at the moment. Why? Bottomline is the familiarity factor. I know it intimately and I've gotten used to using it, quirks and all. For my style of live play, it's more comfortable for me to use the 505. I've spent a good deal of time working with/trying out other bits like the RS7K, the XL-7, the MPC2K, the Electribes, and the RM1x, and for now, I still prefer the 505's ability in the live arena. There's boxes with better sound, better sequencers, and better control, AND maybe they're actually "easier" to work with, too, I just got used to working with the 505.... I'm still trying other things though.
Anyway, the quick answer to your question would be the 505, the "what features" is largely familiarity more than any thing else. If you put in time on any of the machines out there, you'll be able to figure out how to make it work for you.
A lot really depends on how you like to play and what you play. Some boxes are more inclined towards certain styles or approaches of playing than others.
If you've the time, here's some basic thoughts I have on all the boxes I've actually used :
The Electribes are a helluva lot of fun to use and you can definitely do a very "live" set with them. They're excellent for folks who like a modular kind of rig and the individual units all "play" well with each other. They don't have the greatest sound but it is very usable and quite adequate for most things. They're not terribly configurable MIDI-wise, but you have enough control to do what you need to... They are definitely well suited to "jamming" style of play. They are probably NOT a good box for someone who uses or programs lots of notes and other more complex bits. Very raw, very fun...
The
EMU XL-7 is the arpeggiator champ IMO. Friggin' thang has a waaay cool arp section in it, up to 32 arps at once, all synchable, and a lot of ability to modify arps. Realtime record is another cool thing, lets you create or edit sequences while you're playing them. Mad cool sound on this one, best sounding synth engine of the lot IMO. FX are decent, but nothing special. Course with the sound being so nice there's less desire for FX anyway...
It's still a new technology, so it's OS is a little shaky, but EMU have been releasing bug fixes and updates regularly. It doesn't have any means of switching patterns other than scrolling through menus, and it's a MF to edit sequence data. It's actually easier to erase or overdub than it is to try and edit individual notes, although EMU is supposedly gonna sort that in one of the updates... The XL-7 is also a solid box, very well made piece of kit. That's worth considering if you're going to be dragging it all over creation...
The
MPC I ain't gonna say much about. It's got a nice sequencer, it samples... Lots of folk use 'em both live and in the studio. We had one for about three weeks and returned it. Just wasn't flexible enough for our needs. It is a good machine, and it's very good for "grooving" with samples, but I found it's architecture a little stiff and it didn't suit my style of live play too well as I don't really use samples much.
The RS7K/RM1x - different machines but similar enough to put together. They both share a lot of similar features, especially as relates to the sequencer. The RS can sample, the RM1x can't. The RS has a bit nicer sound, a bit more real-time control and nicer FX. The RS is also about twice the price of the RM1x. As to their live playing ability, they're both close to the same, with the RS having a few more perks. Very solid timing on these machines, definitely one of the best sequencers in terms of editing and such. Very flexible in terms of routing MIDI info and a good ability to control other equipment (though the RM1x used to send out some SYSEX stuff that would occasionally freak out machines that were downstream... believe that was only the early OS's/EPROMs though, the updated one seemed fine...).
A cool feature is being able to set your pattern quantize when switching patterns to 1/16th notes. Makes it a lot of fun to "play" your sections. The RS has some very decent FX, and both have some cool MIDI FX that lend themselves well to live use. The RS also has the "LOOP REMIXER" which can give any sequence a new twist, in real-time no less... The weakness of these machines for me is mostly that you have to do a lot of prep work editing sequences to be able to run an entire set without "stopping the beat," so to speak... Not impossible to workaround, but a bit bothersome.
I would like to mention that a lot of the "preset" patterns in the RS are actually pretty decent, and I'm talking about individual sequences or "phrases" as Yamaha calls them. I've found some very nice percussion bits in there for instance. That's a bit unusual in my experience with most boxes, as most tend to have really lame presets....
The
505 is still my favored live box, as I've said already, largely due to me being very familiar with it. As for actual features that I make use of, the biggest thing is it's pattern call function. It lets you set up patterns and variations (including altering every single aspect of a pattern except the actual sequenced data... all patches, levels, panning, fx, etc., can all be modified) so that you can use the same sequence data and switch up sounds, FX, mutes, and even the actual patches, and recall your modified setup at any point in the set. You can move between pattern sets smoothly without any glitches and keep the beat going all night if you want... The Megamix feature is also useful live as it lets you further "mix" up your sequenced bits allowing you to bring sequence data and patch setup from one pattern into another on the fly (like you can switch out basslines, leads or whatever...).
The 505 has also got good control over individual tracks, the sliders, knobs, and so on are well laid out to be able to manage multiple tracks at once, you just need agile fingers. The arp is damn skippy, too. Very flexible and able to be tweaked in realtime, I use the arp as much as I do the pattern call function... Not as fully featured as the EMUs, but still well implemented. I also gotta mention the D-Beam. I first just wrote it off as a gimmick, but after I got to using it I really use it a lot. It makes for an eexcellent alternative controller, you can play scales with it, and it's a massive crowd pleaser. I like it enough I'm trying to get one of the actual Dimension Beams from Interactive Light as they have a lot more configurability, are more accurate, and have a bigger range than the D-Beams licensed by Roland. Anyhoos, I really enjoy controlling MIDI data with them, a lot like using a theramin.
The downside of the 505 is a fairly vanilla sequencer (although the grid sequencer is nice old style Roland...), and weak "out of the box" sound. When I say "weak" I mean that most of the sounds that are preset patches in the 505 (and I won't even touch the preset patterns in this writing as presets are generally lame in any machine....) are a bit dated. They also suffer from a lack of low-end unless you're using the Main Stereo Outs with the low boost feature on. The synth engine IS an abbreviated JV2080 though, so as long as you don't mind doing some programming the issue of weak sound is largely eliminated. The FX are decent, even good in some areas, lame in others. Anyhoos, I obviously had a bit more to say about the 505, but again I've used it more than the other bits. Also, I want to note that a lot of the things I like about the 505 are available in other boxes, too.
All this said, remember, one can get deeper into ANY of these machines than I did, including the 505. Hence, you can't take what I'm saying as gospel, just some opinions based on my own experience and preferences. What has made the 505 so valuable to me is that I spent a great deal of time learning it. I know lots of "tricks" on the 505 that I don't know on the other machines. that's not to say these other critters don't have tricks, too, I just haven't had the time to learn them like I have with the 505.
I'd also like to mention I'm always altering my setup, trying new machines out, making adjustments and practicing. Right now I'm after adding an analog step sequencer and an independent ARP box to my rig. Maybe then I'll "retire" the 505 to the studio for awhile. Then again, I have also been enjoying the RS, and maybe I'll end up finding ways to use it in a live situation that are more to my liking...
One last bit, if you've read this far, methinks you understand there's more than one way of doing this. That includes avoiding these boxes all together. As Influx pointed out, the MMT8 is a very useful live sequencer, not terribly complex, yet more than adequate for most dance bits. That and a couple of synths, maybe a sampler, will take you a long long way. Most important is to figure out, usually with a little forethought and a lot of trial and error, how you want to do a show and buy the equipment that is best suited to that. It may take awhile, and maybe you'll end up like me, constantly refining your kit, but just keep prodding and you'll sort out what works for you. There just ain't no "box of the gods" IMO, least not so far....
Peace and sorry for the massive post....