My recommendation would certainly be AOKP. It's well-documented, quite stable, feature-rich, and has a very large user base. Because of this, you can get used to the custom ROM experience with the same setup that many others use day-to-day.
Give it a spin, and after that you can poke around with any of the numerous ROMs that take alternative approaches to their feature sets. Some are lean, with minimal modifications and every unnecessary component removed, while others are extravagant, with practically every feature included that the developer can think of.