Makes the base game even better. I've had newbies actually complain to me that teaching them the game with Leaders incorporated was the worst possible thing - not because it made it harder to learn, but because then when they go and buy just the base game it felt more dry to them.
Surprisingly competitive play for a game that spends very little time with you directly interacting with the other players. Forces you to make some hard decisions and gamble on occasion to get the big payout.
Lovecraftian mythos in a game that just oozes theme. Great co-op play and for the most part avoids the "busdriver effect" (where one person more or less directs everyone's moves); there's rarely an obviously best option on a given turn, but rather two or three choices that all tend to equally push towards the goal.
My favorite deckbuilder. The central row offers a more tactics-driven game, where Dominion is almost pure strategy; as such, this feels more organic and cerebral instead of the sterilized machine that games of Dominion tend to turn into.
A new "base" set in this series, Storm of Souls continues the mechanics from the previous two releases and adds the concept of Events (think Planechase from Magic) and Trophy Monsters (you pick them up and gain their reward later at a time of your choosing).
Simple game mechanics that betray a remarkably complex play experience. It's amazing how fiendishly difficult it is to maximize your score in this game thanks to the trade mechanism, but that's what makes it so great.
The simple mechanics make it really easy to learn, as well.
A fun co-op/one-vs-many game; everyone starts out playing with a common goal but the game escalates to a point where one player has to work against the others to achieve an opposing objective. A bit lighter than Mansions of Madness or other similar titles while still offering some of the adversarial tension.
A strategy game that plays a bit like Smallworld-but-not. It has a bit of a learning curve, such that a player who is inexperienced at the game runs the risk of backing themselves into a corner or just getting run over by a more experienced player, but it's still a fun and thought-provoking experience.
Another one of my favorites just because of how many people it supports (by virtue of being a party game); it really provokes a lot of creative thought and the card illustrations are wonderfully colorful and imaginative.