now, obviously, this is a really hard decission and i'd rather put up a whole list of good combinations and tactics i've come up with just with the information available at present... but that would take ages.
Don't take this too seriously. Just pick a unit you like the look of!
I must say that the Viking and Baneling look like great fun... but i'm going to go with Colossus! There's just something about having a few really strong units slowly walking around killing everything that is so satisfying in RTS sometimes

and the ability to walk up cliffs is going to give so many fun ambush opportunities and advantages vs some unit types. It will, however, be interesting to see how badly Colossus suffers from being hit from anti-air as well as anti-ground attacks. In SC you could see it having huge problems from cloaked wraiths or scouts which do quite a bit of damage to air... but without knowing as much about the stats of various units in SC2 it's hard to identify the main threats to Colossus from the air. Of course, on the ground, need to watch out for Siege Tanks! Colossus is bound to be heavy armor type which Siege's demolish in seconds. Banelings are probably a cheap way to take them down too.