For defense in this especific Metagame

Dragonite seems to be the best bet here.
Followed by Snorlax and Lapras as always.

If you're looking for the best niche defender... maybe Slowbro? He'd resist both Arcanine and Exeggutor (at least Eggy's quick move).

I like Slowbro as a versatile defender but you can't really go wrong with Snorlax or Lapras (Dragonite's high CP is great but I find their weaknesses easier to exploit).

Whoever has the highest CP :)

I'd power up the pokes you have with the highest max level CP, as long as they are not horrible defenders (like Arcanine or Gyarados).

Hard to argue with Dragonite, Snorlax and Lapras certainly if you got them.