First of all, there's a limit. The vast majority of heads are quest items, so you'll have to actually accept the quest, obtain the head, and then not return the head to the quest giver. This means you're going to be stuck at the limit of 25 quests' worth of heads. How do you maximize your headcount? By making sure to do the quests that result in the most heads. A prime example would be Four Heads are Better than None, a quest in the Twilight Highlands, where you would obtain four heads for a single questlog slot.
There are more than a few heads that won't require a quest slot, since they actually begin the quest. Thus, they're freebies. However, it's still some work to actually get them. Here's the list I have.
- Head of Onyxia (Onyxia's Lair)
- Head of Nefarian (Blackwing Lair)
- Head of Ossirian the Unscarred (Ruins of Ahn'Qiraj - 20)
- Magtheridon's Head (Magtheridon's Lair - Outland)
- Myzerian's Head (Myzerian in Northwest corner of Uldum)
- Gnash's Head (Gnash in Kelp'thar Forest, Vash'jir)
- Hungry as an Ogre! in Dustwallow Marsh. 8 heads.
- Those Aren't Masks in the Abyssal Depths of Vash'jir. 6 heads.
- Four Heads Are Better Than None in the Twilight Highlands. You'll have to quest through a good chunk of the Twilight Highlands to open this up from Lady Cozwynn (Horde) or Cassius the White (Alliance). 4 heads.
- Guardian of the Monument in Blade's Edge Mountains. This is an Ogri'la quest. 1 head.
- WANTED: Cap'n Garvey in Ratchet, Northern Barrens. 1 head.
- Wanted: The Epoch Hunter's Head in Lower City, Shattrath. This quest takes place in Old Hillsbrad in the Caverns of Time.
- Grmmurggll Mrllggrl Glrggl!!! (LOL!) in Borean Tundra. This is around the time where you put on that hilarious murloc suit and save a million baby murlocs.
- Wanton Warlord in Dragonblight. The questline to get this begins with the Emblazoned Battle Horn that drops from Icefist and Chilltusk. 1 head.
- Gurok the Usurper in Nagrand at the Throne of the Elements. 1 head.
- Big Game Hunter in Northern Stranglethorn. 1 head.
- Seeing Where Your Loyalties Lie in Stranglethorn Vale. Many players will not want to do this as it requires you to get Bloodsail Buccaneers to honored, which usually means slaughtering Booty Bay Bruisers and destroying your Booty Bay reputation. 1 head.
- Jammal'an the Prophet in Sunken Temple. Simple dungeon quest. 1 head.
- Hard to Kill inside Magister's Terrace. Another dungeon quest. 1 head.
- Blackfathom Villainy(Alliance)/The Enemy of My Enemy(Horde) in Blackfathom Deeps. Dungeon Quest. 1 head.
- Malas the Corrupter in Zul'Drak. 1 head.
- Ingvar Must Die!(Horde)/Stealing Their Thunder(Alliance) in Utgarde Keep. Yet another dungeon quest. 1 head.
- The Bait for Lar'korwi in Un'Goro Crater. 1 head.
- The Gearmaster(Alliance)/Defeat the Gearmaster(Horde) in Borean Tundra. 1 head.
- The Morons' League (15 heads)
- Headhunting (25 heads)
- Fish Heads, Fish Heads... (8 heads)
- To Steal From Thieves (3 heads)
Ummm... Well, in my research, I've discovered that the Alliance will almost always take you at your word that you killed someone, and in the rare occasion they do require proof, it's usually only one head. So for all you Alliance players out there, if you pick up a head quest, just save the head.
So, assuming you've picked up the quests listed, you've got 33 heads in your bag (plus an additional 51 if you're Horde!). But is that really enough?
Get creative. Try to rebuild an organism using the various components in Azeroth. Get blood, brains, hearts, toes, spleens, livers, whatever you come across in your random slaughterfest. Fill the bags.
Then, open a GM Ticket. This will be the most important part. You have to demand something, and tell the GM to look at your inventory if they think you're kidding. After all, if they check your bags (and your bank if you're that level of depraved), they're going to see some really disturbing stuff. You want to make sure they give you something for being that crazy. Demand an epic. demand a legendary. Demand that they spawn Ragnaros right in the middle of Stormwind. Demand whatever you want. But make it good.
And sweet Jesus, if they actually give you what you want, submit a screenshot. Have fun being wierd!