For the record, no single ROM is "the best"--there is only what works best for you and for your phone. Some ROMs will work for some phone models but not for others, so be sure that you are downloading a compatible ROM. Here are a few of the major players, all of which are downloadable via ROM Manager.

Cyanogen

is the most popular ROM out there, which makes its creators' forum a fantastic place for help and information. It also has the most people working on it, so users get more features and faster fixes. Overall, it's a very good choice for your first custom ROM. Currently it works on more phones than any other ROM, too. For the Droid it comes with an 800MHz kernel (though, as I mentioned above, you can install a different, faster kernel if you want).

Bugless Beast

Less feature-rich than Cyanogen, is generally smooth (on most phones) and stable, but it doesn't have as many options for customization. Development tends to be a bit on the slower side, too. Bootup is quite fast (much speedier than Cyanogen). Under Settings you'll find the ROM Toolkit, where you can access ROM settings and tweaks. It comes with an 800MHz kernel that has many governors. Personally, when I tried this, I missed some of the neat stuff in CyanogenMod, and I had some radio problems and a few initial Market issues, too. The experience was good overall, though.