tldr There's almost always something that's better than Miltank, except in GSC OU. Play GSC OU if you like Miltank.
You didn't tell us the specific format but did put zen headbutt on your Miltank, so I'll assume you're playing a format where zen headbutt is allowed (ie. not GSC OU).
DPP OU: Use Hippowdon or Gliscor if you want a bulky earthquake user, because they get STAB. Use Clefable (better moves), Snorlax (better stats), or Blissey (better stats) if you want a bulky normal.
BW OU: Use Gastrodon, Hippowdon, or Gliscor if you want a bulky earthquake user, because they get STAB. Use Chansey if you want a bulky normal, because it gets eviolite.
ORAS OU: Use Hippowdon or Gliscor if you want a bulky earthquake user, because they get STAB. Use Chansey if you want a bulky normal, because it gets eviolite.
SM OU: Use Mew, Gastrodon, or Gliscor if you want a bulky earthquake user. Use Chansey if you want a bulky normal.
SM UU: Use Gligar or Hippowdon if you want a bulky earthquake user. Use Blissey if you want a bulky normal.
SM RU: Use Gastrodon if you want a bulky earthquake user. Use Porygon2 if you want a bulky normal.
SM NU: Use Rhydon or Steelix if you want a bulky earthquake user. Use Braviary or Heliolisk if you want a normal. All of them hit harder than Miltank.
SS National Pokedex: Use Donphan (STAB), Tyranitar (better stats), Tangrowth (regenerator), or Melmetal (better stats) if you want a bulky earthquake user. Use Chansey if you want a bulky normal, because it gets eviolite.
GSC OU: Miltank is good in this format only if it does something Snorlax can't do, such as this moveset.
Miltank @ leftovers
- milk drink
- growl
- heal bell
- body slam/seismic toss
If you want to use earthquake, then you should just use Snorlax instead. Rollout is almost always useless because it lets the opponent switch to a rock resist and then do whatever he/she/it wants.
