Besides, some might say it is a spoiler to even acknowledge there is another choice, even if you don't say what that choice is. Not me, but some people are like that.
I'll answer this later cuz for my first playthrough I'm focusing on Mario on purpose. Having tried all the others though I'd probably say Luigi.
Loved him in SMB2 as well, and I always use him in Mario Kart. I didn't like what Nintendo's done with Luigi since the GC era, but I like him a lot in this game. I prefer cocky Luigi to pussy Luigi. When he's too whiney and a lil too over compensating-y...I don't like him.
This is interesting, I thought for sure "random" would be the one most would go with. Once I saw all of the characters I knew that I had to mix it up, and random is the easiest way to do so.
I wish you could just pick once at the beginning of the game instead of having to do it every stage though, it only gives you a few seconds to pick it and I forget sometimes.
For the poll, I picked "Random," but actually I don't use the Random option. What I do is simply cycle through the characters one by one. I'll complete a stage with Mario, then the next stage with Luigi, the next with Peach, then Toad, and repeat. I just like to give all the characters their fair share of action, because they all deserve it!
Although now that I'm in one of the final worlds (maybe it is the final world?) (fire), I'm sticking with Mario because it makes sense. Having said that, I do love seeing Peach take down the big fortress/ castle stages (what an awesome role reversal to see her kick ass in a Mario game).