I've played every Super Mario game ever created. I think NSMB (DS) was too easy, but still fun. I think NSMBWii was just as good as SMB3 and I have not played enough of NSMBU yet to comment on it. (I've been too drawn into ZombiU)

I should finish ZombiU this week and then next week I'll be on vacation. So January is when I'll finally get to really dive into NSMBU. But from the few levels I've tried so far, I have no complaints.

I think they are all great games and have yet to really dislike a Super Mario game.

I suck at 2D platformers, so I guess it doesn't really matter. I always rush.

NSMB U Is great with many unique levels, but NSMB2 was repetetive crap.

NSMB series is fine, but Nintendo deliberately makes the music and the visuals far too kiddy. Recycling music and animations from game-to-game is annoying, to say the least. Nintendo has an entire in-house audio studio, they should use it to compose original music for that series same as any other games.

I think a lot of people feel that way about NSMB U.  It definitely appears Nintendo put more effort into that game than the previous three in the NSMB series.

I've played them all except NSMBU, and yes, the old ones were better. Not to say NSMB games are bad though, they are all okay.

Super Mario Bros 3 was brilliant, as was the very first. I never got what's the big deal about Super Mario World though, I've never found that to be one of the best ones. Especially when it gets out-shined tremendously in every single way twice by it's somewhat sequel: Yoshi's Island. Now that was a brilliant Mario game.
Of the new ones, I like NSMB2 the best so far, though I've never gotten far in the other games... The whole 'New' franchise is behind the original series though.

But it doesn't matter anyway, since 3D Mario > 2D Mario .

NSMB is good, but none of them are anywhere near as good as SMB3. They're all far too easy and the soundtracks are dead boring compared especially considering the incredible soundtracks that every other Mario game gets.

I rank them:

1. Super Mario Bros 3.
2. New Super Mario Bros U
3. Super Mario World
4. Yoshi's Island

The other 2D Mario games I am not too much a fan of, in fact I find them fairly boring, and like night and day in quality compared to the above 4. Although there was a time when I thought the first Super Mario Bros was the best game in the world... It probably was at the time.