For MLBs, either prototype or 3-4 run stopper. For DBs, whatever fits your scheme. I personally run more zone (in a 4-3, mind you) and find that's more effective than man coverage. For my SS I prefer a guy who's good against the run, but ideally not a liability in the pass game. But that's just my personal preferences.
I prefer a 4-3 scheme myself because I find you get more sacks that way, and have a greater variety of players available to you. You can have all sorts of different archetypes at DT (as opposed to just a Nose), different archetypes at DE (including converted 3-4 OLBs) and can have pretty much any types of non-34-specific linebackers. Much more versatile IMO.