I had always expressed my praise and compliments for Blaziken, which is, in my opinion, not only the coolest of the three, but also the best to use for me. And also because he's a chicken, which are amazing animals (and before you ask, I don't eat chickens).
Let's see here. Sceptile is a perfect example of a good Grass type, Blaziken is the first uber starter, and Swampert is not only good in the standard metagame, but its basic form, Mudkip, is one of the most popular memes out there, which increases its usage, unlike Seaking. Probably go with Sceptile since it's my favorite of the three.
I like Swampert the best out of those three, because it's a BOSS. Period.
I like the other two, but grass has never really been my type. And I love fire pokemon, and Torchic is really cute and all, but a fighting CHICKEN? bawk bawk! You've got to be kidding me.
Sceptile for me. I've never played Hoenn games, but Sceptile did such a good job on one of my last Gen IV teams I can't forget him. Incredibly fast, and with moves like Earthquake and Leaf Storm, he's awesome.
That, and the awesome design.