Sceptile. Cool design (it looks like a dragon), the king of the forest, STAB Leaf Storm, Swords Dance, yup. Blaziken is ugly, overrated, and Uberz in gen 5 for some reason. Swampert is ok and I like its Water/Ground-typing, plus it's the best Hoenn starter (ignoring the fact that Blaziken gets Speed Boost via the DW), but it's not as cool-looking as Sceptile.
Therefore, my vote goes to Treecko, since it also looks cooler than Torchic and Mudkip. It should be noted that Marshtomp is my favourite middle evo.