I just wiped an iPad, but I'm not sure which enrollment ID to use to re-enroll it.

It really doesn't matter which enrollment ID you use to log into each iPad.  Each enrollment ID can only be used once, but if you wipe an iPad the enrollment ID which was previously used for it becomes available.  You can see a list of available enrollment IDs in Workspace ONE by clicking Accounts (far left) then List View (under Users).  Look for any username with a zero in the Devices column.