Depends on what you want it for. Muay Thai for conditioning and awesome standup and can use a lot of it for self defense. Jiu Jitsu if you wanna grapple but will be very limited on your standup, and Judo if you want to throw people like Karo Parysian does
I prefer Muay Thai and BJJ mixed but am only training Muay Thai for now. Does Kickboxing utilize knees and elbows too?