Mud Slap Donphan

Depends.

Before the change to Razor Leaf (which still generates energy slowly), Counter was head and shoulders the best fast move in PvP. Made Donphan a threat because it did great damage even without STAB.

But for PvE (raids and gyms), it's nice to have a single type for the fast and charged move. Mud Slap/Earthquake is pretty good, if you don't have Rhyperior or Groudon, and Donphan doesn't have the double weakness to water and plant that Rhyperior has, or the RC requirement of Groudon. Not optimal, but a decent value for a weaker attacking type in PvE like Ground.

In PvE it's a better ground attacker than Golem but worse than Rhydon. You decide whether that makes it relevant to you.

In PvP I'd keep Counter, it's still great, while Mud-Slap is underwhelming.