At first, I wanted to vote for Quagsire. However, Omanyte fits the synergy bill better than Quagsire. Omanyte takes the Ice, Fire, and Rock moves aimed at Tropius, while also laying down hazards and being a Bird check. Meanwhile, Tropius can switch in on Ground, Grass, or Fighting moves that target Omanyte. The synergy between the two is great, and as an added bonus, it is the start of a FWG core. My vote is Omanyte.