Blaziken and Lucario are my favorites.
I used a Mankey/Primeape in Firered from before I got my first badge, up until after I beat the Elite 4 for the first time, but I don't especially like that Pokemon. As big a helper as he was, I don't use him anymore. n_n;;
Breloom <3 I adore fungi, and this is the first mushroom-based Pokemon that performs well in competetive battling. My lvl 56 Breloom in Emerald says it all basically.
I love Heracross too, though not as much. I'd probably like it more than Breloom, if it was easier to obtain nowadays.