I must say that there are quite a few overused pokemon from gen 3, i'd say blaziken, swampert, rayquaza, flygon, salamence, metagross, ninjask (with speed boost), gardevoir, breloom and even groudon. Some less obvious, but probably still overused pokemon are altaria, skarmory, aggron, absol, slaking and ludicolo. Although most of the other pokemon are highly underused, for example kyogre is amazing, it has pretty much the same movepool as groudon just with different types, and the types really work to kyogre's advantage, plus sheer cold is the only OHKO that will work on any type. I don't think i've seen anyone use a mawile, torkoal, grumpig, solrock/lunatone, glalie or tropius plus several others, they really deserve to be noticed!