This is an easy choice for me - Super Mario Bros 2. (USA) any day of the week. That game is outstanding, others come close but it's definitely my favourite 2D platformer in the series. A 2.5D game with the same graphical style as NSMBU would be perfect - have Mario, Luigi, Peach and Toad playable again with the same attributes they had in the original; have it take place in Subcon; Birdo, Mouser, Tryclyde, Robirdo, Fryguy, Clawgrip and Wart can return as bosses with some new ones thrown in too; vases; all the enemies from the original can return such as Phantos, Shy Guys and Snifits can return with some brand new ones too; make the combat the same with you beating enemies by tossing them at each other or tossing vegetables at them; same life-bar as the original with you entertaining sub-space through potions to get mushrooms to increase it... and so on. The game can further be expanded with multiplayer co-op play (being able to chose between Mario, Luigi, Toad and Peach is much better than Mario, Luigi and two Toads), each world having a SMB3 style map, more worlds, more levels in each world, maybe even have a new kind Yoshi (has to be a new kind since it's Subcon) ridable.
I'm excitedly tapping away here, but the game would be perfect. IMO the 2D Mario series has became very stale with them all being too similar, I mean, how many times are we going to have to jump on that flagpole, fight the Koopalings, beat Bowser and rescue the Princess? The first NSMB was cool because it was a great nostalgia trip and the first 2D Mario in something like 12 years, then NSMBWii expanded on it which was cool as well, but now we have NSMB2 and U which look to pretty much the same thing, I can't help but feel apathetic about them for that reason - it was cool at first but now the novelty's worn off. SMB2 stands out like a shining beacon because it's so unique and different to the rest of the series, yet unmistakably feels like a Mario game. A 'revival' game would be the same for the NSMB series. I don't think it'll ever happen though, too many idiots who've never played call it terrible for not being a 'true' Mario game, which is bullshit. SMB2 doesn't get hyped non-stop like 1, 3 and World do either, so Nintendo probably see it as an unpopular game, hence why it's rarely referenced, which means they'd be afraid of making a new game based off it. Also the perfect name for it 'New Super Mario Bros. 2' is gone.
Re. the other poll options:
Super Mario Land would be my second choice, like SMB2 it has so much uniqueness going for it - the SHUMP style levels, original and exclusive enemies and bosses, Daisy, a different villain, unique worlds (such as an Easter Island themed world)... it dared to be different. Many people (including myself) inevitably grew up playing SML too with the Mario series being so popular and the Gameboy perhaps being the most popular console ever, so the game does mean a lot to many people and is much more iconic than many people realise. A NSMB styled game based off SML would be perfect. I'd say the exact same thing about SML2.
Mario Golf... there's no reason why it shouldn't be brought back, but I can't say I'm that bothered about it.
Yoshi's Island already has been revitalised, it got a proper sequel only a few years ago as well as pseudo-sequels in Yoshi's Story; Yoshi Topsy Turvy; and Yoshi Touch and Go. I would however like to see the series take the next step and go into 3D with a 3D Land style game i.e. traditional Yoshi's Island style platforming in a 3D environment. Also don't let Artoon anywhere near it.
Other games... new versions of Wario's Woods and Yoshi's Cookie would be cool, so would a new Wrecking Crew, they'd make good eShop games.