It can be either, as it has good HP and Defense, but I think that it serves better as a Tanky support, Heal Bell being a very useful support move to heal your team from bothersome burns and poison. If you max out Special Defense it will be able to take both physical and special attacks well, allowing it to heal the team. I recommend this set:
Miltank @ Leftovers
Ability: Thick Fat
EVs: 252 HP / 4 Def / 252 SpD
Calm Nature
- Heal Bell
- Thunder Wave
- Seismic Toss
- Milk Drink
Seismic Toss us to deal some decent damage to your opponent, Heal Bell and Thunder Wave are support moves, and Milk Drink is to heal Miltank.
Edit: I just saw your tag for in-game, didn't notice it before. OK, so if this is for in-game, then EVs don't really matter, so you don't really need to do anything, Miltank is naturally bulky, and so all you have to do is decide your set. Because there isn't much strategy or stalling in-game, a support set might not be that useful, as battles are more fast paced and attack orientated. There is no problem in having one though.
