I have some very clear rankings.
1.)
Super Mario 64
This is definitely my favorite Mario game. It's a complex 3D platformer that's not exactly a translation of the classic gameplay--but it's certainly the second-best realization of the Mario universe in history. On top of that, the exploration elements shoot the replay value through the roof (here it has an edge on every Mario game but Sunshine, which has
more exploration but isn't the least bit fun to explore anyway). Mario's acrobatics feel smooth and are also necessary in the 3D environments; you won't be jumping on enemies much, though you'll still be platforming throughout. This doesn't play exactly like the old Mario games, but it does feel just like them, and sometimes you'll respect how well it all translated to 3D. Not only is this my favorite Mario game, it's my all-time favorite 3D platformer. Period.
If you want the most bang for your buck, get this game. I'd still say it's worth $50, though you can buy it for $10 on the Virtual Console. I'd recommend either the DS or the original version; both bring a lot to the table. While the DS version does offer more levels, I don't really like playing as the other characters. I still prefer going solo through Peach's castle as Mario.
2.)
Super Mario Galaxy
This one's second-best to Mario 64, but that doesn't mean it's any less impressive. In fact, it benefits from its comparative linearity and lack of exploration; it has more of an artistic feeling, and the platforming elements are more pronounced. I honestly think this is Nintendo's equivalent of
Fantasia. The music is impressive (
here's a sample), the worlds are, like Mario 64, expansive, and the gameplay is almost seamless. It does stumble in a couple areas (the gimmicky Wiimote minigames don't always work so well), but the final product is so polished you can excuse whatever deficiencies it has. In some ways, it has even more polish than Mario 64--though that does not make it better.
This is Mario's epic. Somehow, Nintendo managed to take the original concept of a plumber rescuing a princess from a turtle and, without changing much about it except aesthetic, make it an exhilarating, even occasionally emotional experience. As far as I'm concerned, this is the single best game on any console of this entire generation.
3.)
Super Mario World
What makes this the best 2D Mario game? It's simple. It takes the grand aspirations of Mario Bros. 3 (which is, I hate to say, not among my favorites) and tosses in the quality level design of the original Mario Bros. Instead of the short, random bursts that characterized the third entry, you get longer, more involved levels with more refinement. On top of that, there's a sandbox feel to the whole package from which you can draw a straight line to Mario 64. Play and replay levels to discover secrets, look for new exits, fish for items. Some levels remain hidden altogether unless you're clever and thorough, and you're usually rewarded for discovering secrets.
Unlike alot of other people, I don't have much of an interest in Yoshi. But it is noteworthy that this is his first game. To be honest, this is almost a "greatest hits" kind of game (ignoring for a moment the existence of Super Mario All-Stars): anything good about 2D Mario is here. And since Mario-style platforming is still best experienced in 2D, this can effectively be termed the definitive conventional Mario experience. Like Mario 64, you can't go wrong with this game.
4.)
New Super Mario Bros.
There isn't much I can say about this game. It's a lot like Super Mario World, but it feels simpler, and doesn't quite have the quality level design. It's more gimmicky, like Super Mario Bros. 3. In this case, those gimmicks work quite well, and the levels are still long and creative. It is on the easy side, the new items (giant mushroom and turtle shell) aren't all that useful except, in the former case, for scoring extra lives, and Bowser's been replaced by Bowser Jr., a stupid and ineffective villain. For these reasons, it's not as good as Super Mario World, but don't be fooled; it's still a fun Mario experience that manages to be even better than the original. Highly recommended--just don't make it the only Mario game you ever play.
5.)
Super Mario Bros.
This was the first videogame I owned. I spent many long hours with it, usually dying around World 4, but it was still a whole lot of fun. The level design in this game was so effective that I'd argue, within the Mario franchise, it was rivaled only by Super Mario World. Its simple design never felt bogged-down; you just advanced from level to level, stumbling upon rare secrets when you could. I still think the best way to play this is on the original NES with an NES controller. The overhaul it got on the SNES made it feel like a completely different game, and I really liked the 8-bit soundtrack.
This is an obvious recommendation. The only reason it's so far down on the list is because Nintendo's been very good at improving upon the formula, and two of the 3D games are both impressive enough to outclass all the 2D entries. It's still worth playing, and it's a nearly flawless experience.
6.)
Super Mario Land 2: 6 Golden Coins
This plays a lot like Super Mario World, except it offers you more freedom and is, as a result, a whole lot easier. You can enter almost every world from the start. Mario controlls smoothly, though more slowly than he does on his console counterparts, and the graphics are as good as anything you'd see on the SNES, they're just in monochrome. The music is also different and familiar at the same time, with a forgotten but still iconic
theme. The amount of character in this game is refreshing. It's one of the best games you can find on the Game Boy, and easily the most underrated game of the entire franchise.
7.)
Super Mario Land
Playing Mario on the go had to feel great back in the day. That said, this is a short game with generic level themes (it still has a strong soundtrack) and decent level design. Not enough to rival its console brethren, but it didn't have to. A solid translation of classic Mario gameplay to the handheld.
8.)
Super Mario Bros. 3
I'm recommending this because it's pivotal to the series and everyone will tell you it is "among the best." I find the level design practically nonexistent, and placement of things feels random. Furthermore, there are a lot of half-baked ideas that wouldn't come into full fruition until Mario World, and the sheer number of suits is overwhelming and feels contrived. This game deserves credit for helping to invent (along with the original Mario Bros.) the aesthetic we now associate with Mario games and tossing out some interesting ideas. It is extremely overrated, and it's not nearly as good as the original Mario Bros. It is still not a bad game.
9.)
Super Mario Bros. 2
Not bad, but not particularly great. Now part of the canon. Its gimmicky gameplay was fun for awhile. Whether or not you like this is a matter exclusively of personal taste, so I won't dwell on my opinion of it. I haven't played it as extensively as I've played every other game on this list, so I might not be giving it enough credit.
10.)
Super Mario Sunshine
I'm loath to call this a Mario game, as it feels like even less of one than Super Mario Bros. 2. FLUDD ruins platforming. It's beautiful, though, and it's not bad; like Donkey Kong 64, it's just overdone. I do not recommend this game.
I think that's all the major ones. XD Sorry to type them all out, but I tend to get carried away when it comes to this series. Can't hurt to have definitive rankings. 1-7 are all
highly recommended, 8 is recommended for its influence, 9 is recommended for its quirkiness and Sunshine...is vaguely playable.