I voted Seige tank, yes it is overused. But it helps alot... I use it too(I'm not exactly overboard with it,) When using seige tanks its like a 50/50 chance of winning the game(if you are playing 1v1) because if your opponents go ground(happens to me alot), massing tanks at the entrance of your base will kill all of them because you have too high range, but then, if they go air(happens to me even more), i'm just dead with me struggling to make some goliaths and turrets in time. But in 2v2,3v3,4v4, zc, I mass tanks in the middle entrance so that they are not able to use ground, while my allies do the air-killing for me, then I kill their base.
But yes, seige tank IS overused.

[right][snapback]163026[/snapback][/right]
Then just make some Goliaths at the same time as Tanks, so you don't 'struggle to make some goliaths and turrets in time.'
It is used in every game, but if it helps it is not overused. It is if you make a lot of one unit that can be easily countered (i.e. going mass valkaries is overusing them.)
I have no idea as to most-overused... probbably Carrier, though.