Making a new top 5 as I've played many more games now:
5 - Mario Tennis: Ultra Smash
Great gameplay can't save you when you don't have many options to play it. Ultra Smash had some neat new mechanics that still bring me back from time to time (hey, Aces didn't have Jump Shots). But there just wasn't enough content here to justify the price tag. The only single player content here is the Knockout Challenge, which is basically just copy-pasted from the previous installment's StreetPass matches. That's it; there's no tournament or adventure mode to speak of. Multiplayer isn't exactly great either, with only one rather bland minigame, weak Wiimote controls that give the GamePad player a rather major advantage, and no option to play online with friends. There are a few saving graces that secure this at only #5, such as the Mega Mushrooms, Jump Shots, and playable Sprixie Princess, but it's definitely not the Mario Tennis series' finest hour.
4 - Super Paper Mario
This list wouldn't be complete without the Most Overrated Mario Game of All Time (TM). The story is completely devoid of typical Mario charm and humor, the character direction is lacking and relies too heavily on new OCs, the gameplay is repetitive and just downright mediocre, and overall this game feels so disconnected from not only the other Paper Mario games, but from the Mario franchise as a whole. Easily the weakest Mario RPG by far (and contrary to what many others will say, Sticker Star is a far superior game).
3 - Mario Party: Star Rush
I'll admit, I had high hopes for this one. But after the fantastic Island Tour (it was a great game, sue me), Star Rush just felt lacking. Toad Scramble had a lot of different board layouts, but they all just felt so similar. Balloon Bash is one of the blandest experiences I've ever had in a Mario game. And everything else was just a glorified minigame. Speaking of which, the actual minigames were rather lacking in both quality and quantity. I couldn't help but feel disappointed by the overall mediocrity of the package. But despite this, this is not the last Mario Party you're seeing on this list.
2 - Mario Kart Tour
You know it's a bad sign when a Mario game, usually known for its family-friendly nature, dips into the realm of aggressive monetization. Dr. Mario World was at least somewhat generous in its random character system, but Mario Kart Tour is very stingy in comparison. Rubies, required to gain new characters and vehicles from the pipe, are exceedingly rare, there's a hard limit on how much EXP and coinage you can earn per day, and--worst of all--200cc requires a $5 monthly fee. Yes, you read that right. There's no good excuse for any of this. The gameplay doesn't exactly make up for it either, as manual drift is impossible to use effectively without the somewhat iffy gyro controls. Combine all that with a significant lack of original tracks (with the ones there are being rather bland), no Luigi as of this posting, and a complete lack of any multiplayer options make this one of the worst installments in the Mario Kart series.
1 - Mario Party: The Top 100
You have a great idea, you severely underutilize it, and then you sell it for $40. That's just what The Top 100 does. The highlight of the Mario Party series has always been the minigames, but the decision to make that the sole focus proved to be very disappointing. There was a party mode, but it had only one board and it was just copy-pasted from Star Rush's Balloon Bash, of all things. The rest of the package is really just an expanded minigame mode. The minigame selection leaves something to be desired, being horribly unbalanced between games and having a few notoriously bad stinkers in there. The single-player mode is fun, but that's about all The Top 100 has going for it. This is the worst Mario Party by far, surpassing even the depths of Star Rush.