Pelliper for an easy early game (bulky, useful typing, has mist to get rid of op field effects, good against anything not named Julia early on)
A bit later on that has to be Archeops for me, thing is crazy useful. Rock slide hits hard in general and is especially good for for all the double battles, acrobatics (later fly+leftovers) oneshots anything that doesnt resist it or has high defense, endeavor for pulse/boss battles or when it survives on low hp, and knock off for getting rid of pesky items and oneshotting psychics and ghosts. As of ep 16 it can even learn stealth rock.
Edit : forgot about goldeen/seaking, goldeen is actually really good early on with lightning rod (lol@ julia) and when it evolves it learns waterfall which is very strong at that point in the game, that mon made my last playthroughs early game a joke.