...Would be to get through all my games without using Legendary Pokémon. So far I've managed this on Red, Yellow, Silver and Crystal (I've sold all four games since) - currently trying this on HeartGold, Platinum, White and Black 2.
So far I've noticed that my Black 2 team is the most capable and can tackle pretty much any team thrown at me.
Team consists of:
Lucario - my main Pokémon
Crobat - I always have one on every team
Mamoswine - needed a tough Ice type AND Ground type
Starmie - mainly due to high speed and ability to learn Thunderbolt, not to mention I needed a fast Water type and Psychic type
Volcarona - one of the toughest Fire types, and my favorite Fire type AND Bug type
N's Zoroark - this guy is temporary, I had him since I needed a Dark type
Proof of this team's toughness is the fact I beat Alder's grandson while my Pokémon were four levels below his. It was tougher since I couldn't use items (like Revives and Potions) in that battle. XD That, and strategy works. My Mamoswine owned those dragons (it actually OHKO'd Iris' Haxorus with one Ice Fang).
PS. I will probably have to give up on this goal when Pokémon X comes around. I like Xerneas way too much NOT to use him.