Oooh...that's a hard one haha. I have so many lol. There are many good, reasonably priced heat protectants out there. But since you have a CHI flat iron and curling iron already, I think you would absolutely LOVE the Chi Silk Infusion Reconstructing Complex (it contains no alcohol, so it won't dry your hair out and it doesn't buildup). Not only is it a great protectant (three major silicones are listed as the first ingredients: cylcomethicone, dimethiconol, and dimethicone), but it has silk proteins which will penetrate at the maximum into your hair by using the ceramic heat from the flat iron but the high silicone barrier prevents the actual heat from damaging your hair (cell membranes let certain things through and other things they do not). But bottom line, it reconstructs and strengthens the hair while protecting it from heat damage, so that's one reason I love it sooooooo much.
I also have Nexxus Heat Protexx (really good), JOICO Reconstruct Leave-In Protectant (it's supposed to prevent thermal and environmental damage), Garnier Sleek and Shine Weightless Anti-Frizz Serum, to name a few.
So yeah, if I could only choose two out of all these, I'd buy CHI Silk Infusion and the Nexxus Heat Protexx. If I could only choose one, CHI Silk Infusion. You can find it at Trade Secret, JCPenney salon, etc. etc. I bought one of mine from JCPenney and the other two from folica.com.