Your question isn't all that clear, but I'm going to go ahead and assume you mean "Which Pokemon has the most Evolution Lines?".
In which case, the answer is very obviously Eevee. Eevee is able to evolve into 9 different Pokemon depending on the conditions it finds itself under. Stones, Happiness, certain locations, etc.
If you meant "Which Pokemon has the longest evolution chain?", then there is no clear answer. Pokemon seem to have evolution chains maxing at 3 tiers, so the longest chain is always going to be 3. Of which there are a great many.