I like TTYD more but the original is fantastic as well. I think TTYD's humor is a cut above which makes it stand out, and the partners get more development for the most part than they did in Paper Mario. They're both pretty easy games overall (though some parts are a lot harder if you don't understand how the battle system works, like Huff 'n' Puff). TTYD had some annoying plot-related tasks (the aforementioned chapter 4 and the things involving Frankie and Francesca). Paper Mario's environs were a lot more colorful too which I liked, but TTYD has a few more... unique areas. If there's one area where I feel TTYD falls short, it's music. It's not bad, but there are only a few tunes that I specifically want to listen to from the game. Paper Mario has a lot more, and I believe SPM has even more.
People rag on Super Paper Mario because it didn't have the same battle system but I think it's just as good as the first two, just different.
Bobbery is the bomb, BTW.