My most desired combo is Fire/Grass. Seriously, when GF revealed Cherrim and Flower Gift, I could swear she would evolve to a Firey cherry tree (I think it isn't the right term, but...), with Flower Gift/Chlorophyll. Pimpin'.
The idea of a Fighting/Ghost always attracted me. Maybe a ninja pokémon, with high speed, average defenses/HP and above-average Atk/SAtk? Deoxys-S syndrome, but he wouldn't be uber anyway.
Steel/Grass (Carnivine and Mawile evo?) is sexy. Neutral to Earthquake, Ice Beam, Megahorn, blahblahblah... Only Close Combat and Ember to worry about.
Masquerain evolving and regaining is Water/Bug type would be its redemption. And I believe it's a good attacking combo.
Ice/Electric, as someone said before, for the Boltbeam STAB and resist.
Fight/Flying for Farfetch'd? STAB'ed Close Combat and Brave Bird would wreck anything at sight.
Fire/Dragon, Electric/Dragon, Ice/Dragon, Grass/Dragon and Steel/Dragon too. Fire is obvious and nullifies the Ice weakness; Electric for a nice attacking combo (I think only Steelix resists Elec+Dragon); Ice for massive special damage; Grass because of Power Whip and the support moves nearly every Grass learn, and access to Draco Meteor for its counters (much like ChainChomp); and finally, Steel because Dialga shouldn't be so selfish, Palkia even let Kingdra borrow his type and all... And it's the one of the most defensive types (or THE most defensive) that can be created.