Mordekaiser: Those ghosts and his damage shield... uhg, he is so boss. I like to dodge Karthus's ult by using my Siphon on minion waves :3
Veigar: He just owns so hard. If the enemy team isn't stacking, and I do mean STACKING, Magic Resistance... he just nukes them. I've twoshotted so many people, mostly AP champs, with his ult/baleful. And the limitless AP he can generate off minion kills is just plain amazing. Had over 150 ap from just my baleful strike farming one time. Add his huge stun, his massive damage aoe spell... he is a teamfight god in my opinion. Cage and drop dark matter, ult/q down the squishies, proceed to q/w until you can no longer cast spells... usuually you find that the enemy team is gone.
Twitch: Need to gank someone? Ambush them, Expunging if needed or if you just wanna see their HP go byebye really fast. Need to get away/help your team get away, without putting yourself into too much trouble? Dehabilitating poison from the longest range you can, and boom, you're prolly home free. Same if you need to chase/help chase. Enemy team bunching together like fools? Ashe can hit five birds with one arrow... Twitch can hit five champions, several minions, a tower... you get the point. Wanna burst down a tower for luls? Ult, ambush, rinse and repeat as needed, and that tower will probably not survive. If he wasn't so squishy, he would be OP as heck.