Parasect & Heatran are the definition of covering eachother's types.
Parasect is weak to:
Fire (6X with Dry Skin)
Flying (4X)
Ice (2X)
Bug (2X)
Poison (2X)
Rock (2X)
Heatran is weak to:
Ground (4X)
Fighting (2X)
Water (2X)
Parasect 4X resists Ground, 2X resists Fighting, and is immune to Water with Dry Skin.
Heatran is immune to Fire with Flash Fire, 4X resists Ice and Bug, is immune to Poison, and neutral to Rock.
Unfortunately, I've found Parasect isn't exactly "a force to be reckoned with". Adament with Spore/X-Scissor/Screech/Pursuit paired with Timid Heatran with Overheat/Earth Power/Dragon Pulse/Hidden Power [Ice/Electric] can take down quite a bit, though.