I don't know about "best," but if you want to see a list of commonly used gems for various purposes, I've found The Ruby Toolbox to be a great resource. The rankings are determined by counting up the number of forks and watchers of various github projects, so I'd view it less as "this is what I should be using," and more as "these are some things I should check out." At the very least, they're all likely to be under active development and fairly up to date, and it's very useful to see groups of gems broken down by category.

To get a list of all gems from a particular source, you can use gem list --remote, as August points out. Rubyforge (which is your default source) and github are definitely the two most-used.