Mudkip.
Why? It is one of the few Water types which, when evolve, will get Ground as their sub-type.
Moreover, Swampert is an OU Pokemon compared to Blastoise and Feraligatr, which are both UU. Em...Emp...Pochama's evolution has a rather weak movepool like the two UUs, and perhaps the furthest in can go will be BL.
Also, remember that the other two Water/Ground maximum evolutions, Whiscash and Quagsire, and much weaker than Swampert, being NU and UU respectively, and low Special Attack statistics.