Only two! They can be from any system, and they can be old/new doesn't matter. The ones that you completely loved and sank countless hours into! :)
For me, i had to think long and hard, because i have played quite a few. They are:
1. Crisis Core: Final Fantasy VII (PSP)
2. Star Wars: Knights of the Old Republic (XBOX)
Oh and also, if you like, post why you loved them so much or what sort of emotional connection you have with them. i.e, one of the first RPGs you ever played, amazing story you cried over, phenomenal gameplay that kept you coming back for more, ect...