I love all of the Mario RPGs. Each one of them is great and wonderfully made.
SMRPG: New characters, a new villain, a great story, fun battle system, creative places, you can't stop playing the game.
PM: Simple battle system, felt like you were really there in the Mushroom Kingdom, colorful, simple, and classic.
M&L: New kingdom, lots of twist and turns for a fresh game, challenging battle system.
PM2: Fun chapters, cool and unique places and characters, Bowser is playable.
M&LPIT: Neat graphics, aliens, babies, Kamek, past, fun enemies.
Notice that most all of these aspects are the same for every one of the games. This is why Mario RPGs are so fun!