What is the largest fish that my 30 gallon tank can comfortably hold? I'm interested in larger fish that I can more visibly see and maybe interact with.

In a 30 gallon I typically wouldn't go any larger than 5" or 6" fish.

So for example: angelfish, firemouth cichlid, krib cichlid, convict cichlid, rainbow shark... fish around that size are the maximum.

Those examples shouldn't all be housed in one tank, of course. But those are some good individual centerpiece options. OR a pair of convicts would be okay in there.