Paper Mario series... Man I know too much about those games lol.
If you have not played the first one, you are officially shamed by DMG. I don't care how you get your hands on the game (cartridge, Wii Download, whatever), you need to play the first one. Excellent Storyline, progresses nicely, good partners, great concept overall. The bosses are a joke ONLY if you understand fully how to manipulate the tools given to you. Otherwise most of them can be extremely challenging in a good way to defeat through "normal" fighting means. There are plenty of side quests and additional tasks to perform if you want to obtain or truly "perfect" the game. Plenty of badges to fool around with, items to use, cooking recipes to use, etc. You get to explore everything from the desert, to a Chilly Northern Mountainside, to a Tropical Island with a Volcano, to a Toy Box full of Shy Guys, To Flower Fields, etc. You will not be disappointed with that game. There are few genuine flaws in this game.
The second one, TTYD, is also pretty good. I found the plot line/partners just a bit too similar to the first one, and it was easier to be "broken" in that game, but it also had improvements over the original Paper Mario. Beating the final bosses is not the last thing to do (you can keep playing on after you beat them to do other things). Additional badges like W and L emblems are nice. Overall still a good game, but I feel the first one is still better overall. Honestly if the first one had just a few corrections, it would be one of the really close to "perfect" games out there.
The third one, Super Paper Mario, is a hybrid platforming RPG move around and do weird stuff game lol. It's not bad, but it did disappoint a lot of people who thought it would follow a bit more closely to the first two in the series. Nonetheless, taking the game at face value for what it is, it's certainly an acceptable addition to the series. The change in the battle system is understandable and the same with item changes, but it made it much harder for Mario and crew to realistically lose or even take damage even against the strongest of bosses. Especially with the help of Barry, you could beat just about anything in the game by playing a game of hit and run smartly. Dimensio, Shadoo, etc. Honestly the hardest thing IMO to complete is arguably the 100 man Trial at Sammer Kingdom because of the sheer number of enemies you have to prepare for and play so long perfectly to not take enough damage over time to lose a life. Even that can be done if you are a patient person without too much hassle. I also felt that Luigi was added to the crew WAY too late into the game. Getting to play as Luigi was great, but by the time you got to play as him the game was almost completely done. I mean you could say the same about Spike in the first Paper Mario, but you still had to beat the Crystal King AND go to Star Haven AND Maneuver through Bowser's Castle before the game was over so it was acceptable to get him kinda late. Luigi on the other hand, IDK. If he was available even a single chapter before then, I think that would have been much better. Maybe that's just me though.