That's sort of like saying "I'd like to buy a vehicle. Which one should I get?"
Read AnandTech, ArsTechnica, Tech Report, Storage Review, dozens of hardware review sites that I can't even remember (and their associated forums in many cases) and see what's popular, high performing, and/or low-cost, depending on what youre requirements are.
Five hundred dollars can get you a pretty nice setup if you've already got the video card, sound card, accessories like mouse, keyboard, speakers. Even if you need video and sound, integrated motherboards can be cheap and good performing.