Technically, the highest level Pokemon are gen 1 glitch Pokemon that go far over level 100.
But for non-glitch Pokemon, the highest ones catchable are Magikarp and Gyaados. Yeah, Magikarp. There is a small chance of catching a Magikarp at level 100 using a Super Rod in the Resort Area in Platinum, or the Nature Preserve in B2/W2. As for Gyarados, there is a small chance of catching a level 100 Gyarados at the Nature Preserve with a Super Rod, like Magikarp, but in Rippling Water.