Someone you get on with and can trust.
First ME took me too literally and overdid the bass, would have preferred some push back. My second ME was someone the band found and he _should_ have told me to go back and fix the top end balance of my mix. It wasn't that either of them were not technically proficient, but with my lack of experience and a lack of relationship we didn't get a good a result as we otherwise could have.
After that worked much more closely with a couple of other guys in the same mastering house, talk early, sent samples over before the session. Attended the beginnings and ends of the sessions to make sure we were all on the same page (the middle is messy). All in all a better result.
That said, do you need an ME? No, but if you can afford it and want a commercial (or club) release then they have their benefits, both in quality assurance and making albums sound whole (if/when you have an album).