My Experiance is limited in this area but I can tell you that the combo between an SGX-2000 express (800 programable channels with alot of pre-sets) using a
Rocktron Intellifex includes a built in HUSH (reprogramable, download's also available via the internet) on the stereo channel is un-friggin-believeble for the shear number of sounds and combinations available..... All controlled by the ART X-15 footboard which is total Midi and has 2 variable and seperatly programable foot peddle controllers...
All 3 of these unit's are less than $400 in the good used market....SGX 2000 Express figure on around @ $200 The Intellifex probably @ $250 and the X-15 vary's between $125 and $175 for one in excellent shape..
Digitech is also a wonderful product although I do remember the earlier one's Pre-21xx series had a few problems with their boards (since then they have improved greatly).
The all out best unit IMHO you can put in a rack really isn't what I would call a multi-effects unit it's the
BBE Sonic Maximizer. I had a 422A in my rack because I was running stereo but all in all your going to notice alot of dynamic punch with that one little unit no matter which model you purchase....Plus they are generally under $150 used in excellent condition....
One side note

on one of the programs in this combo I have literlly hit a note and gone to the store came back ate lunch and went back to the guitar and it was still playing that same note (sustain forever LOL I don't remember the channels I was using but it was a combo of preset's) Kicks the gibson family to shame sorry all you Les Paul guy's but that was with an RG 550 and standard pups..Yup same volume and tone till I muted the string just to prove a point to a freind about rack stuff.....
Just my .02 for what it's worth.....