Mario & Luigi: Bowser's Inside Story. I haven't played most of the other games on the list though...

Paper Mario 1. Edges out TTYD due to perfect soundtrack.

PM1 > TTYD > PiT > BIS > SS >>>>> SMRPG >>>>>>> SPM >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>> Sticker Star

Woops! Didn't read the thread title properly lol Favourite RPG has to be Bowser's Inside Story.

My favorite is Super Mario RPG...
Followed by Paper Mario and Paper Mario TTYD...
I have a confession to make...
I've never played any of the Mario and Luigi titles...

This was a tough choice but out of all of them... Thousand Year Door takes the cake. The best Mario RPG ever! I wish they'd do a real Paper Mario on Wii U, then all will be forgiven.

     I say that you should play every Mario and Luigi game except Partners in  Time..It's a huge borefest.

They're all excellent, except for 2, which are merely "quite good": Super Mario RPG and Sticker Star.
Though I find it hard to classify Sticker Star as an RPG, to be honest.
Favourite would be Paper Mario: The Thousand Year Door, followed by Mario and Luigi: SuperStar Saga and then all the others, though I really can't come up with a concrete ranking.

So, in short:
PM:TTYD>M&L:SS>M&L:PIT/M&L:BIS/M&L:DTB/PM/SPM>>>SMRPG(/PM:SS)

Paper Mario: The Thousand Year Door also happens to be my favourite game of all time.

   Sorry, dude, but Mario RPG is WAY better than Sticker Star..  Music is better,  characters are better ( Bowser barely even speaks in Sticker Star), battles are more entertaining, there are  more secrets and optional things to do like the casino,  Super Suit, Attack Scarf, the Culex fight, Jinx, etc...

SMRPG felt so "un-Mario" to me. Maybe has something to do with Squaresoft doing the game, but I don't really know. As an RPG, SMRPG is superior to Sticker Star, but that's only because I don't think of Sticker Star as an RPG to begin with. In the end, looking back at my time with both games, they were both quite good and I both enjoyed them just the same, which is why I put them in the same category. One might as well be better as the other, but that opinion changes so frequently in my mind (I can't, for example, decide if I like Mario&Luigi: Bowsers Inside Story or Super Paper Mario more: One day I'll say SPM, the other it'll be Bowsers Inside Story) that I don't want to list them in concrete order. It's just that I think of both as "quite good" games.

        Maybe because Square worked on it  along with Nintendo back in the day? it is still a good game to me.