Ok, I guess to get this straight, how big of Monsters are we talking about?
We looking for stuff like this?
http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat640011a&prodId=prod1250012a
or would this be big enough of a monster?
http://www.games-workshop.com/gws/catalog/productDetail.jsp?prodId=prod1320015a
For scale size, that little goblin bundle thing in the Ogre box is pretty small, but the Ogres are about twice the size (and girth!) of a human. The good thing about the Ogre size monsters is that at least 2 armies (that I can think of) can use them as basic troops, meaning they can fill the army with them and not require ANY human sized models.
Beastmen: The Ghorgon/Cygor (one kit), the Jabberslythe and the Giant are the big Monsters here. Monsters Infantry include the Infamous Minotaurs and their hero/lord equivalent the Doombull.
Daemons of Chaos: All of their Lord choices are monsters, but that's about it. And all their lords are EXPENSIVE (cheapest is 300 points) so you'll probably only have 1 in most games. They have Monstrous Cavalry in the form of Blood Crushers and Fiends, but not much else...
Dark Elves: Manticores, Dragons and even evil Pegasi for the lords. With a truly frightening Hydra in the Rare slots. Pretty nasty monsters, decent variety.
High Elves: Can field characters on Dragons in both the Hero and Lord slots, and are the only army that have hero monsters (pretty sure...) They have Eagles and Griffins for Monstrous beasts. The Griffon is also part of the latest Stater set and can be easily found on Ebay, if that is a good enough Monster for you. As another slight nod, High Elves also have Lions pulling Chariots, which can be awesome in it's own right. XD
Lizardmen: Carnosaurs (basically T-Rexes) and Stegadons can be used as mounts, with the Stegadons also being in the Special AND rare choice. Kroxigars fit the Monstrous Infantry, and can be taken in Core alongside Skinks (the exact OPPOSITE of Monsters.) Also smallish monsters in the form of Slamanders and Razordons.
Ogre Kingdoms: The Premiere race for Monstrous Infantry. Ogres abound in all slots backed up by "2" monsters (1 kit). The Stonehorn and Thundertusk are nasty beasties, but outside of The Mournfang Cavalry (who are BIG) these are the only MONSTERS. But seeing as the rest of your army is still BIG "infantry", you'll dwarf most opponent's.
Orcs and Goblins: Lords can take Wyverns, which are lesser Dragons. You can have goblins riding BIG spiders as Core, but the Spiders aren't monstrous, just big spiders.

The Aracknarok Spider is one of the biggest creatures, and can be also used as a mount. So, if ya like Spiders, got plenty here!
Skaven: The Hellpit. Frightening and disturbing, but the only thing among hordes of Skaven. Rat Ogres make up some decent Monstrous Infantry and can TECHNICALLY make up all most of your armies core with a specific special character, but still, lots of small rats are key here.
Tomb Kings: Sphinxes! War Sphinx and Necrosphinx are shiny and really tough monsters, followed by other Monstrous constructs (though are just Cavalry and Infantry, spectively) of the Necropolis Knights and Ushabati.
Vampire Counts: A Zombie Dragon, Terrorgeist, the Vargulf and 2 new Monstrous Infantry make for lots of Monsters. They also got the new, shiny Coven Throne and Mortis Engine. Not Monsters, per say, but big and shiny models that are beautiful and nasty.
Warriors of Chaos: Chaos Dragons, Manticores and Shaggoths dominate the Monsters, supported by Dragon Ogres, regular Ogres and Trolls (which can be Core with Troll King Throgg!) as a nasty backup. Also, they have some of the hardest, toughest infantry in the game: the imposing Chaos Warriors. Monsters in their own right.
Wood Elves: Treeman and even a Forest Dragon can make some nice and unique monsters, back up by Treekin (smaller Treemen) as Monstrous Infantry and even Dryads for a Forest Spirit theme can make a thematic and cool looking army. Lots of Finecast though, in case that bothers you. (it can be expensive and fragile)
That about covers it. There are a few others, but they aren't worth mentioning. The Empire has ONE Dragon, who can only be used by ONE special character.
And then there is Storms of Magic, an expansion for special games that use even MORE monsters for EVERYONE, including huge spells and wizards. LOTS of monsters in that book, and once again, they're available for EVERYONE.
I'd look over all the models, and what you'd like from the rest of the army. If you want nothing to do with puny humans, Ogre Kingdoms and Warriors of Chaos (using Throgg) can do just that! If you want the Monsters to stand above the rest of the army as a major focal point, then perhaps Skaven, any of the Elves or even Lizardmen focusing on Skinks will do the job of large variance in size. As you may have noticed, some armies have more variety than others. Lizardmen have lots of different monsters, but they're all Lizards (duh). And Ogres have all Monstrous Infantry and Monsters, but they're all Ogres or Mountain themed monsters. High Elves only have Dragons (not counting the Griffons, Eagles or Lion Chariots). So, pick your poison. Monsters are abound!