Magma, for: Reasons already given. Unlimited fuel, cheap n' easy superweapon, yadda yadda yadda.
DANGER ROOMS, for: Reasons already given. Admittedly, danger rooms are almost 100% necessary considering just how slowly dwarves train their skills (even with full-time training and no inbound threats, no skill ups even within a year,) but they're so overpowered. Yes, I understand that Toady must have implemented wooden training weapons for a reason, no I don't understand why dwarves train so unreliably without them.
Here's one that nobody's said yet: Giant olms. Holy shit, they just bite goblins right in half, at least the only times I've ever seen them on the field anyhoo.
Armor is vital to survival in real life, so that isn't really overpowered, and it's only realistic that two people of equal power and skill and armor and arms would take forever to kill each other. Walls aren't overpowered, since they do exactly what they're supposed to do. Danger rooms are unrealistically good at training dwarves, and magma is also way too easy to use and manipulate compared to how it is in real life. If things like magma-smelters could exist in real life well okay, but they don't. Magma is hot enough in real life that it should really even kill dwarves and other creatures that come within a few unseparated tiles of it. I've never played with magma within easy access, personally, mostly because I'm more concerned with things like getting a high biodiversity at start. I almost always start on a tropical moist broadleaf forest for that reason, and since I don't play with adv. setup parameters, I don't usually get volcanos on those tiles. I also like water more than lava anyway because it's harder to weaponize, and it's also more realistic. I really don't like this whole player mindset that magma immediately makes everything cooler... even though it usually does.