As of what we know now, here's the answer:
The highest is Audino, at base 390,
And the lowest is Magikarp, at base 20.
Audino is found by going up and down Route 6 (they sometimes jump out from bushes). You can also battle it in Cafes and the Battle Chateau.
Magikarp is found by fishing at Parfum Palace, Route 3, or Route 22.