The best ground Pokemon is almost definitely Swampert. It can become ground type and learn mud shot at level 16, which is easy to reach before the second gym. Before the sixth gym, you can get the Swampertite and raise Swampert's base physical attack to 150. Only 2 ground Pokemon (primal Groudon and mega Garchomp) have a higher base attack stat, and neither are obtainable nearly as early as Mudkip. This is the moveset that I would recommend.
Swampert @ Swampertitie
EVs: 100 Spe
- earthquake
- waterfall
- stone edge
- ice beam/hammer arm
If you don't want to use Swampert for whatever reason (such as the fact that grass starters are always the best), then I'm pretty sure the next best ground Pokemon is Camerupt. Fire ground is a good offensive type combination, and Numel can learn powerful moves like lava plume, earth power, and overheat pretty early.
Camerupt @ expert belt
EVs: 100 SpA
- overheat/flamethrower
- earthquake/earth power
- stone edge
- yawn/eruption
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