Level 50 Statistics (see level 5, 50, 100)
Miltank is good if used correctly, which basically means not using Thunder Wave or Seismic Toss; Blissey does this far better than Miltank does. If you must use Miltank, use its natural advantages over Blissey or don't use it at all.
Curse is probably Miltank's best option. With Milk Drink and Curse, it can hold its own against the likes of Salamence and Blissey; however, watch out for Skarmory.
Psych Up lets Miltank steal Dragon Dance boosts from Tyranitar and Salamence, Curse boosts from Snorlax and Swampert, as well as Calm Mind boosts from Suicune.
Thunder Wave and Seismic Toss are viable moves, but as previously mentioned, Blissey uses these more effectively. Shadow Ball enables Miltank to take care of Ghost-types.
Checks and Counters
Skarmory, Choice Band Metagross, Gengar, and Celebi are all solid counters, while Snorlax, Swampert, Suicune, Milotic, and Vaporeon work against Miltank that use Seismic Toss and Thunder Wave.