To buy or not to buy: Su-35, Su-30MKI, or Su-37?

Well, I was going to suggest the original Su-27 as it is very significant in history. However, since you are also planning on getting the MiG-35 instead of MiG-29, I assume you don't want a design that is "too old". In which case, I would suggest Su-35. The rationale is a simple one: the Su-35 is in Russian Air force whereas J-11 variants and Su-30MKI are not.

Su-35. The Su-37 is a nice tech demonstrator, but that's all it is.