Here is a Poll someone did on the forum several years ago showing 64% use RobotC. I would even assume that this number has grown since then but I can’t find newer data.
Recently someone put together a list of who they thought were some of the most fearsome VEX teams.
The top 4 all use RobotC. This is just off the top of my head so in all likely hood most of the other top 10 teams use RobotC as well.
If you are doing a team where education is a big priority than I would highly advise RobotC as it will give a more realistic coding experience than easyC without raising the required knowledge to start. EasyC is nice if a team doesn’t have a dedicated programming member and only wishes to make simple code for driving the robot.
RobotC even has a nice documentation/community/Technical Support guy named John. 