I use a few rods. #4 for crappie (depending on the average size) and bluegill, #6 for smallies in rivers that don't have other "large" species in them, #8 for bass and top-water pike/muskie work, and a #9 (which is more of a #10) for the deep down and dirty muskie/pike/big game work.
However, If I could only have one rod for warm water, it would have to be the #8. The ability to turn a large grass carp before it runs into the brush is essential. This includes large mouth bass and large smallies as well. Another factor is that in warm water you're often fighting moss. That extra rod weight helps with that, for sure.
Now, this is just for rods. If you have line questions, that's a whole different ball game.