There's the fun party, card game called Munchkin, then there's its 8+ different expansions, then there's another 10+ different thematic decks, each with their own expansion sets, all which can mixed and matched for various games. It's a bit of a headache to actually think of them all.
I've got plenty of experience in the world of Munchkin, and I've spent far more than I'm willing to admit. I'm making reviews for each of the decks I own, as well as their expansions as I go.
Original Munchkin Deck
It's the very first Munchkin deck, and by default, one of the best. It set the standard for everything to follow and offers a good variety of cards, a stable balance, and some game changing cards. It's a deck that would be recommended for anyone even if they have no intention to invest in more thematic decks or expansions.
Munchkin 2: The Unnatural Axe
The first expansion to the original Munchkin deck, The Unnatural Axe adds in a new Race, the Orc and several new cards all around. It's a nice breath of fresh air after playing the original numerous times and it doesn't over-saturate the game or unbalance anything. That being said, it doesn't address many existing problems in the original deck either, few as they may be.
Munchkin 3: Clerical Errors
The second expansion to the core Munchkin game, this game adds in two more character modifiers, the Race of Gnome and the Class of Bard. It also adds in a level 20 Monster, the Kali which can easily become the most powerful Monster in the game just by appearing, and some very interesting cards largely designed towards the Cleric, a pre-existing class that's debateably overpowered. In other words, people who hate other people who play the Cleric class should like this.
Munchkin 4: The Need for Steed
This deck expansion adds a brand new type of equippable to your characters in the form of Steeds. This deck is overloaded with Steeds, Hirelings, and their respective enhancer cards and honestly, not much else. The expansion causes the game to become a little more watered down where monsters become more rare but easier to handle.
Munchkin 5: De-Ranged
Did you not get enough Steeds in Munchkin 4? Maybe you miss having an overpowered class like the Cleric? Maybe you just want more moments where you have nothing to fear from the monsters!
If you like Munchkin because any of the reasons above, you and I are in disagreement. But if you do like the suggestions above, you'll probably be interested in this expansion with a powerful new class that creates new Steeds on a whim and has almost nothing to fear from any Monster.
Munchkin 6: Demented Dungeons
This is probably my favorite expansion in all of Munchkin. Dungeon cards are a side deck, and by far the best. While some other themed Munchkin decks have their own Dungeon cards, this one takes the cake (possibly because it's the first, but also because the Dungeon of Gender Bending). You will have lots of fun with this, and even better, it fits pretty well with the base core game without expansions, or with most other Munchkin themes.
Munchkin 7: Cheat with Both Hands
A necessary contribution to a bloated deck, or to allow two different thematic decks to combine for a single game. This expansion is pretty bad on its own, but it compliments bloated games extremely well.
Munchkin 8: Half Horse, Will Travel
This is the seventh and currently final expansion to the original Munchkin fantasy deck (there are other decks that are made to be mixed, but for purposes of chronicling these expansions, this is the last in this line). However, this might be the worst expansion yet, since it simply adds more of the same to what already exists, doing nothing to fix the oversaturation issue of the game. Completionists will grab this expansion, everyone else should probably stay away.
Finally venturing into a different thematic deck, Star Munchkin is a breath of fresh air with brand new cards (although most of them fit nicely into familiar slots). If you've invested a lot of time and money into another deck so much that it's become unwieldy, Star Munchkin is a well balanced game that will bring you back to the good ol' basics.
Star Munchkin 2 - The Clown Wars
Quite possibly one of my least favorite expansions, The Clown Wars adds a lot of nonsense, undesirable cards, an unintelligent and confusing card type, and nothing exciting. I sincerely hope you don't waste your money on this one, but if you like, you can read my review on just how poor this expansion is.
Star Munchkin - Space Ships
More of a booster pack instead of an expansion, this adds a new card element called ships to your game! Unless you already have Munchkin Booty that is, which incorporates said cards much better. Still, Ships can be a lot of fun so this little pack might be worth investing in.
I not only have more than enough Munchkin decks for the game, but I also create a variety of Munchkin custom cards as well! You can gloss over the majority of them, getting a quick snippet of each card.