Poison / Dragon
Psychic
Grass / Ghost
If we're look for effectiveness in BW OU, note that the only Pokemon that resist all of Fighting (for fighting spam and Terrakion / Virizion / Scrafty in general), Water (for Rain), Electric (for Rotom and Thunder spammers in Rain), and Fire (for Sun teams) are Latias and Latios. Both however have the misfortune of being weak to U-turn (Scizor is a top 5 used Pokemon) and Pursuit (Tyranitar is a top 5 used Pokemon), two crippling issues which often prevent players from easily switching either in during competitive play.
A Poison / Dragon gives us the only Pokemon ever that would resist Fighting / Water / Electric / Fire / Bug and be neutral to both Pursuit and Stealth Rock, meaning, in this BW OU metagame, it could switch in much more easily than expected and carry out whatever its Sketch strategy is. The ability to switch in allows it many opportunities to set up should we decide on a defensive or support build.
However, Poison / Dragon is additionally attractive in that the Dragon STAB opens up interesting offensive options as well should we decide to go that route; Quiver Dance with Draco Meteor (to OHKO things) or simply Dragon Pulse (to sweep) or Coil with Outrage / Dragon Claw / Dual Chop both would help this remain viable offensively, and an admittedly poor offensive typing in Poison could help us make sure this doesn't sweep with reckless abandon considering a Hidden Power would have to be spent on either Fire or Ground (just don't give it a coverage move).
I think it's important to remind ourselves that this only gets to Sketch once...I'm seeing a lot of questionable "x typing combination will make this broken" posts and with what we have at the moment, this is a really unnecessary fear. Smeargle has access to unlimited sketching and it is only barely UU. I know Smeargle won't have nearly as good a typing combination / stats as we will give this, but it is important to note that even unlimited sketching doesn't break a pokemon (or make it even OU). This is only sketching once; the moves that could potentially break it have to be used in conjunction with other moves, which we can easily take care of in the movepool section (Shell Smash with Baton Pass, Quiver Dance / Coil with too many coverage moves, Spore with all the entry hazards etc.).