I can suggest several, with various observations on each. However, for a first time ROMer, I'd suggest something that doesn't have a history of bugs, OTA nags, and various other issues.
What are you looking for in a ROM? Stable, fast, and feature-rich? Minimalist? Stock-like? Tweaked to all heck and gone?
CyanogenMod 6.1.2 would be for the former, MetickMOD for the next one, CM again for the stock-like, and if you want tweaked then look at Ultimate Droid but be warned every new release seems to add more bugs to the mix. But there are a whole slew of others. If BB had a better history I'd have no problems suggesting it, but the Dev is a good egg. BB v.5 required some tricks to install it that if not followed would cause major issues with the device. It also has some great community support (themes and the like).
UD is a perennial favorite here, with great community support and more themes than you can shake a stick at, but it always seems to have one major bug after another. I don't recommend it for that reason, but it may be just what you're looking for if you don't mind screwing with y our phone. CM is created by a group of people calling themselves Team Douche with good reasons -- but the ROM is fast, stable, filled with more features than you'd believe, there's a new nightly posted every day if you get bored and want to see new features, and most other ROMs take at least SOME code from CM. MetickMOD has an attentive Dev, it's very minimalist, and when there's a problem he's on top of it ASAP.
Simply Stunning is a great ROM too, actually, but it might be geared a bit more towards geeks than the average user, and MIUI is awesome in its potential but can be a beast to install.