Defender Variety

That Lapras mmmmmmm.
I have one but it never makes it into a gym - but you know they need to dedicate one attacker to your Lapras defender that they wouldn't usually take.
It is just a shame that CP is a thing.

Maybe wait until we see if there is a gym update - it may change your decisions.

Slowbro, Lapras & Steelix are decent defenders, but will always sit in the lowest rung. Worth it if you have enough stable gyms.

Espeon & Scizor both have high enough CP to last for a while, but their low stamina makes them comparatively weak. They are often taken out before they can fire their first charge move.

Donphan is the only real defender in the list. Tackle is sub-optimal, of course.

psychic defenders are always tricky. if you don't dodge quick moves, you get hit really hard. A Flareon can loose against a psychic exeggutor defender, even with typing advantage.
Espeon is psychic-only so it's only weak to dark (maybe another type too?). And dark is weak in general. So if you don't dedicate a umbreon to attacking a gym with espeon, the attacker will loose a lot of HP.
Fighting type is weak to psychic too.
So Espeon makes a good defender I think and psychics with confusion in general are ok. Ice move on Slowbro may be useful too.