Its actually pretty simple. The Droid, Incredible, and Nexus One are all running Android 2.1; the Eris and all the older phones are running 1.6 or 1.5. Pretty much anything new coming out on Verizon is 2.1.
There are a ton of roms out there; but they give you a lot of flexibility. Even there, there are two extremely popular custom roms - bugless beast and cyanogen. Both permit overclocking, and provide a very fast Droid experience.
The Incredible gets good reviews, although I am not sold on the SenseUI; sure, some of their widgets are cool - I just hate the curved Phone button built in, that is there regardless of what you are doing. I just want the full screen. But the Incredible's hardware is pretty sweet. 1GHz processor with 512MB of ram built in.
I am looking forward to Droid 2 (which I hope won't come out until I qualify for a annual upgrade). Same basic phone, more power. That's my hope.