Chemistry isnt just about romance. I think AM and TW also have great chemistry, but I've never seen it as romantic and I dont think I ever will. Their acting comes across very naturally and they both seem to have a real genuine understanding of each other, both in character and acting.
I saw loads of romantic chemistry between Clark and Lana, but the easy going friendship chemistry, due to the way their relationship has been written, was always awkward. However, TW and KK were able to pull both sides of that off due to the chemistry both actors have in being able to work with each other so effortlessly.
Now I also see great chemistry between TW and ED, they are able to play off each other in any given situation. We've not yet seen any romance between their two characters, but I can easily see it being special because of what's been developed between the two so far.
Aside from my opinions though, chemistry is always a very subjective thing. You have to be totally unbiased towards characters/actors to be a true judge of whether its there or not.