I use them all.
Coach Suggestion if I know from experience the play I want will be there (fastest way to get to it).
If I know it's not there I'll use Formation.
If I'm not 100% sure of the formation the play is in (was that Gun Trey Flex or Gun Flex Trey?), I'll use Concept.
Finally, for some not so common plays (FB run, QB draw, etc.) and play action, I'll go to Play Type (mostly habit).
Ideally, if the playbook was pared down and I knew it as well as I should, I'd use Formation almost exclusively (and perhaps Suggestion if I knew the desired play would be there).