mooooove over
Miltank is a solid OU in GSC, a status it never really achieves again, likely due to the mediocrity of its typing and defensive stats that just aren't world-beating in later gens.
In GSC, it fills the niche of physically defensive Heal Bell user. Unlike Blissey, it can tank Curselax and even stall it out with Growl (Bliss gets Growl too, but instantly dies even at 100% health to a crit Double-Edge). With Growl, Miltank is also a great answer to stuff like Tyranitar, Rhydon, and Nidoking, walling what are otherwise very scary mons pretty easily. Miltank is also super-fast, so it can even try to abuse Speed Ties against Tentacruel, Charizard, and Zapdos, and it outspeeds everything else (that fast Milk Drink is important when it comes to walling mixed sweepers like Nidoking). Miltank is also capable of using STAB Body Slam for general paralysis support and damaging capabilities, and could use Curse to make the opponent actually consider falling back to a more passive mon (although replacing Growl with Curse means that the hardmons [Steelix, Tyranitar, Rhydon] can set up on it with Curse + Roar instead of being countered). Bliss, meanwhile, needs to rely on having the correct super-effective elemental attack and is woefully incapable of actually doing any direct damage to Snorlax.
Basically, Miltank is a good all-purpose check to Curselax and a wide assortment of other things on top of being a user of Heal Bell.
You could use other attacking moves over Body Slam (I mentioned the pros and cons of SToss in another thread), or you could even eschew attacking in favor of something like Toxic, but the fact remains that Body Slam is standard because it's spammable in the greatest number of situations. Historically, Miltank has also used Curse (already discussed) and Psych Up (you live a +6 Bslam from Drumlax, so you can use this to try to counter-sweep) in place of Growl. Coverage moves like Earthquake and Shadow Ball are also (barely) viable if you're really paranoid about Ghosts, who otherwise are moths to Miltank's flame. In general, though, Growl is Miltank's best option because it lets it check the most stuff.
Blissey's Heal Bell is a lot easier to pull off, though, as Electrics are everywhere and Blissey has absolutely no problem switching in and tanking them. By contrast, most of the mixed sweepers that Miltank easily switches into are not as common, and the one thing it tanks that is mega common (Curselax) can be difficult to switch into (34-40% damage on the switch is no laughing matter). Regardless, Miltank tends to synergize better with a team because the things it checks are very different from the things Raikou and Snorlax are already well-equipped to handle, so it tends to get the nod.
I should mention that I'm not personally a huge fan of using Miltank because, while it does check a ton of things, it checks most things passively with Growl (thereby making it very vulnerable to crits), and it doesn't get that many chances to use Heal Bell, which is its main purpose.