I voted Tracks / Wheeljack, and here's why.
Sunstreaker / Sideswipe was obvious. I mean, they may not have been the same toy in G1, but they both had similar alt modes and they were supposed to be brothers, anyway. So it makes sense that they would share a mold. You may as well have included the Seeker mold in your poll, as far as I am concerned, if you were astounded at how well a shared mold worked for these two characters.
Tracks and Wheeljack on the other hand? My first thought on hearing that they would share a mold was, "Ooohkay... that's kind of a wierd choice." I thought the designers were being a bit lazy. But then you get them in hand, and it totally works. Both characters manage to look distinct, and have iconic elements of their original selves. (Not really related to the mold itself, I'll admit, but the Reprolabels really help both Tracks and Wheeljack out, too, I'd have to say.)
Shameless self promotion time! I don't have a review up of the Sunstreaker / Sideswipe mold. (I have been meaning to write one... I've declaired April to be "Hot Rod Month," but maybe I'll do it for May's first review...) But if people want to see my further thoughts on Tracks and Wheeljack, as well as see some photos of them both together, I have a review of them on my website
. End shameless sell promotion! (Well... except for my signiture...)