It depends what type of pokemon you need...
Milotic is a good special sweeper with a really good moveset.
You can give him surf, ice beam, aqua ring and attract... like this you have 1/2 chance to not be hit this turn and also you will be healed...
Munchlax can be really a deadly pokemon if he evolves. Giving 252 Atk EV and 252Hp and Belly drum, rest (to cover belly drum's damage), sleep talk and E-quake/crunch/giga impact etc. can be really deadly. Both pokemon are good but personally I'll go with Milotic because Munchlax needs many time to set up...