IMO, the best intro RPG is now out of print. Basic D&D is how I got my start, back in the day. The new D&D now reproduces the feel of MMORPGs in its mechanics. If your focus is on combat, this may be a good or a bad thing, depending. But if your focus is on the story and the roleplaying, it's fine, and easy to learn.
If you want versatility, I'd go with GURPS, from Steve Jackson Games. The rules are a bit more complicated, and character creation is a bit bookkeeping-heavy, and min-maxing is easier than I'd like, but you can take a GURPS character from a fantasy campaign and drop him into an outer-space campaign and not have to do any rules conversions, which is a wonderful thing. If you're willing to help new players through the character creation process and the combat stuff, GURPS is a good starter system.