I'd say the ones that are on them are the best ones.
I don't see the point in spending a bunch of money for new heatsinks for 2 low end video cards.
To be blunt: I'm not interested in your opinion of the quality of my graphics cards. I asked what a good aftermarket cooler for them would be. The 550 ti's rank in the bottom of the high end cards or top of the mid grade cards. I suggest acquainting yourself with information before you post. Please refrain from off topic responses in the future.
Thanks for the handful of answers that give me a jumping off point, though none quite solved the issue, because no data was solid.