I use Rototags, the normal and the electronic ones. I buy them from Dalton. It's true you have to buy 50 from them but then they don't eat anything and can be used the year after and the year after.......
I've found Rototags the very best for actually remaining in the ears. Tags with loops seem to get caught, you can cut the loops but then why not by tags without, and ones with thin pins through the ears seem to tear the ears more easily.