The 4690K and 4790K are the most popular on the Z97 mobos (you don't mention which one, there are numerous Z97 mobos from Asus) in the ATX form factor. The K models are OC'able, with the i5 4690K aimed more towards gaming and general purpose work, the i7 4790K is also a good gamer, and has Hyper Threading so geared towards higher end computing uses