Highly debatable topic... you'll probably get recos for everything from the mega-Axess Electronics unit to the Behringer FCB1010 and everything in between.
Personally, I really like the old DMC Ground Control units... relatively flexible, easy to program, phantom powerable, generally durable/reliable, and can be had on the cheap ($100-150 used depending on software version and condition). I used one of these with my RM4 rig (and another with my Triaxis rig back in the day). The Rocktron MIDIMate is close second in terms of features, ease of use, and price... I'm currently using a MIDIMate with my M4 rig.
--B