A ground type would be advised for coverage against most of Team Galactic, so Drama's advice of going with a Hippopotas when you get it would be good. You could otherwise go with a Shellos as you have access to one now, though the team is getting shafted of that 10/10 rating down to 7.8 when there's too much water
Hippowdon is quite slow, however, so you may want to find a faster Earthquake user. Gallade may make a potential one for coverage with Psycho Cut, Close Combat, Energy Ball and Earthquake seeing as how the only one on your team that brings hard-hitting fighting moves to the table is Staraptor with Close Combat (and not until reaching evolution), and you find Ralts earlier than Hippopotas with the former being at Route 208. Overall, getting that ground coverage is the priority, so going for Hippopotas/Ralts as the permanent member and having something like Shellos fill the role until then would work.