Heatran and Slowbro. They're both just so amazingly useful, and irreplaceable. I just have to add Amoonguss or Mega Venusaur for a great FWG core.
Offensively, Breloom, Tornadus-Therian, and recently Mega Manectric are my must-haves.
I run my Breloom without Spore most of the time; I prefer all-out offense, and prefer the utility of Mach Punch, Drain Punch, and Rock Tomb. Burns are far too useful for the physical juggernauts.
Tornadus has a good level of bulk, ridiculous sped and power, plus great utility with Knock Off and U-turn. Regenerator is god-tier, and Hurricane's 30% confusion chance is clutch.
Mega Manectric is actually kinda similar to Tornadus. I run Thunder, and it's surprisingly powerful, with great neutral coverage. The Speed tier is amazing, and Overheat + HP Ice gives Manectric useful coverage. Intimidate + Volt Switch stacks well with burns, and it makes an all-round excellent revenge killer.
