How about that one part where they totally skipped Mouser in World 3 and replaced him with Mecha Birdo in Super Mario Advance? =w=;
Considering that there were a grand total of 3 Mousers in the JP original, I guess it only made sense that he would eventually get more downsized along the way (though he was just moved to another level, as Shelly said). Even so, that third Mouser from the original game (5-3's boss) was removed to make room for Clawgrip, one of the first "GIANT ENEMY CRABS!!!!" in gaming, so that's fine.
If you have to know, I support all 4 of them.
When I play a full game, I tend to switch to each character on each level.
Mario first, then Luigi, then Peach and then Toad in that order.
That way, when the game is over, all of them has 5 points in the ending. ^^
My lady! That's the way I usually tried to do it too! Being fair and all that.
But in any case, if you really want to blaze through the game, in the quickest way possible, all you really need is Toad, actually.
Toad has the unique property of not only lifting things better than any one else, he also goes FASTER, the heavier the item is! With the right item, he's both the fastest way to get around on foot, as well as through those annoying digging passages.
If all you want to do is clear the game in about a half-hour or less (using warps), Toad is your mushroom-man. And if you got either Newtype-esque, pixel-perfect timing (or TA-ing through an emu), you can make it so Toad can get double-jumps to give him more maneuverability. Then that just makes him even better.