honestly: none of them
quagsire's attack is far superior to its sp.atk. mud bomb is a special move, has shabby accuracy and low base power. earthquake has 100% accuracy, 100% base power and makes use of quag's attack. if you were to choose one of them, use mud bomb but seriously: teach it earthquake. no chance of reducing opponents accuracy drop, but everything else makes up for that. here is a good move set to build around it:
quagsire @ leftovers
trait: unaware
impish nature
EVs: 252 hp/ 252 def/ 4 sp.def
moveset:
- earthquake
- waterfall
- recover
- stockpile/ curse (prefer stockpile to stall, but its opinion based
this set is mainly made to stop set up sweepers with its nice ability: unaware. use stockpile a couple of times and recovery when needed. as I said: physically quag is much better that special, but I can give you sp. set too if you really want to use mud bomb:
quagsire @ leftovers
trait: unaware
bold nature
EVs: 252 hp/ 252 def/ 4 sp.def
moveset:
- scald
- mud bomb
- toxic
- recover