Depending on the venue really. I use 24-70 f2.8 and sometimes a bit tight if shooting adults in a small ring. If lighting is ok shoot low down but watch out for the ropes. If lighting is crap ask to use flash but again watch out for the ropes. The referee will get in the way so quick tip is if the fights are being filmed position yourself on the same side as the video man as the ref may (and only may) try to keep out of his way.
Another tip is I always set focus point to the right of centre and focus on the fighter to the right, this enables me to keep the fighters more centred as I dont have time for cropping before the see the images.