I voted for "Clowns" because it is the most end-game content you can arbitrarily trigger to "win". Allow me to explain:
1 - FPS death isn't an intended feature and is by far one of the most subjective aspects in the game. It can wildly range depending on your computer's specifications, so this means you can have a FPS death with 50 dwarves or play normally with 200. You can also have FPS death because of a pathfinding bug or game programming issues (e.g. the game handling data the hardest way possible making it consume an unecessary amount of memory). So FPS death should be out of the table, and I hardly believe lots of people consider it to be "win". They usually abandon because they get pissed about not being able to do anything, or they force a "fun" situation.
2/3 - Losing spectacularly is considered "fun" and win by most people. Actually, this might be my personal favorite. Losing because of a legendary megabeast that was sleeping for years under your fort until you dig it up is as satisfying as it gets. By a certain point of view, this is also the only way of "winning" with the first option, because you made your fort last so much long that it will have to be spectacular to make your fort end. It can be frustrating if you were really engaged into a personal megaproject and it ended up making you lose your fort and progress.
4 - These are extremely fun but it usually is the most frustrating way to lose. Because it is so "easy" to set up defenses (you could literally just close up your fort, tell the invaders to fuck off and live without even needing to open it up), there are literally infinite ways of dealing with these. It is much more satisfying to release a sea of magma into the invaders than losing to them in a "fair" fight.
5 - Yeah that's win, but your fortress won't die. After it you'll end up doing something else or relinquishing your fortress anyway, in which case you'll be missing the whole point of the game.
By point of the game I obviously mean making epic (and !!EPIC!!) stuff and bragging about it on the forums more than you would if you had 100000 gamerscore on LIVE. Also having infinite times more recognition for it, and eventually becoming a legend to never be forgotten by the community.