If there was an award for 'best mario game' then it would be a real close call whether to award it to this or the SNES (and now also gba) follow-up, Super Mario World. This being my first time playing SMB3, I must say the first few hours of play were somewhat underwhelming, especially due to the lower level of eye candy compared to SMW, as well as the simpler level design and less responsive controls. also what bugged me was that i couldn't revisit completed worlds and would often be scraping through as small mario and never really getting enough of the various suits the game had to offer. thankfully, later on the levels become more interesting and give more opportunity to don the various suits (racoon, frog, tanooki, hammer bros as well as the one-off 'giant boot' which can be used to jump on usually deadly enemies). really later on the game becomes extremely challenging (more so than SMW) and due to no mid-level checkpoints and not being able to go back and stock up from old levels, these can become extreme tests of patience - but once bowser is defeated you can revisit any level from any world and suddenly the game really blossoms as the same level of freedom as SMW becomes apparent.
all in all, i'd say that i like this game just as much as SMW but for different reasons. SMW is a great overall package and probably the best all-round mario game to date, but without SMB3 alot of the elements in SMW would not be refined as they are. SMB3 is a more challenging game with fun(er) powerups, but on the flipside, the boss fights and the graphics are quite primitive.