I love XCOM's Second Wave options almost as much as I love XCOM itself, which is quite a lot. Second Wave is a suite of design choices that were added to XCOM: Enemy Unknown in order to make the game harder, less predictable, a bit more weird. All of this, though, at the discretion of the player. Second Wave put a lot of power in the players' hands.
Powers like Damage Roulette, which does exactly what it sounds like: weapons would suddenly inflict a wide variety of different damages. New Economy, meanwhile, did something very similar for XCOM funding - you might get huge amounts of money from the council, and then you might get pretty much zip. Elsewhere, Not Created Equally gave rookies a random selection of initial stats, which made a surprisingly massive impact on the game, moment to moment. Onward and outward - those are just the first three options Second Wave offered.
I think I love Second Wave not just because I love chaos, and because some weird, difficult part of me wants a cruel game to be that bit crueller still. I love Second Wave because it feels like a great compliment to the player to be offered it. You, Second Wave suggests, are a player of great creativity and attention to detail. You're in the kitchen up to your elbows in spaghetti sauce. You leave Post-it notes to yourself about home bio-hacking projects and you do your own translations of the classics. None of this is remotely true of me, but I want it to be true, and fiddling with Second Wave makes me feel a bit more discerning, a bit more detail-oriented. It's gorgeous stuff.