I love them all equally for very different reasons, but the funny thing is I nearly passed them all up. I rented and played though much of Mario RPG in the late Snes days, but I did not buy it used and beat it until the 64 days. Initially I was not going to touch it at all because it seemed too silly and I had been playing Final Fantasy III and Chrono Triggger incessantly for some time.
Paper Mario I picked up used about two years ago or less when I saw a copy for $30 at Hollywood video, and Mario & Luigi I almost passed up because I was already well into Fire Emblem and Sword of Mana was about to come out.
I do not intend to make the mistake ever again. Paper Mario is my most wanted game after Final Fantasy 12 and Suikoden 4, and it comes out well before.