It all is VERY dependant on the foci and what you want to do.
example 1: I want to be an air/off shifter?
I'd like combos like bowlder-hawk/hummingbird/tiger or crag/falcon/tiger. I am fine if you change the tiger with lion.
example 2: I want to be a def/off nexus?
dragon/croc as defensive and wolverine/bear/rhino for as the offensive. If I get sand lizard, I'd prefer armadillo over any other form for my defensive. (Maybe I should include silverback here, but I never had it to know how it works)
For a def/def, I would love to have one of pachyderm/porcupine with a region form preferably croc.
The question is not correct I think. Maybe you meant which forms you like best and why?