What martial art would you train in alongside Boxing; Brazilian Jiu-Jitsu, Freestyle Wrestling or Taekwon-do?

I'd choose Brazilian Jiu Jitsu, but my opinion is obviously biased as I train and love BJJ. But then again, I believe that BJJ is a very technical martial art unlike some other types of wrestling. I also believe that BJJ would make a good mix for a stand-up art, although I'd train in Kickboxing instead of just Boxing.