I think the most approachable RPG would be Mass Effect 2, it's a fast paced shooter with lots of RPG Elements. It has all the ingredients, but simplified compared to real RPG's. But i really can't recommend it before Mass Effect 1. Knights of the Old Republic is one of the greatest RPG's i have ever played, but it's probably a bit dated. Fallout 3 or NV is a good choice, because you can tackle the game however you want(FPS,Melee etc.), but it might be a bit overwhelming. Personally i'd recommend New Vegas, for me it was much easier to get into. Dragon Age is absolutely a fantastic RPG, with a great storyline, but if you never played RPG's it can be a bit punishing. Though if you understand the mechanics it actually has a quite tactical and fun combat system.

These are i think the easiest to get into, because they are pretty new, very good games aimed at a more mainstream public.