Oh hey, it's this debate again.
The Dark Project is the one I played first and the one I've played the most, and its first five levels are probably the best first five levels I've seen in any game, just fantastic in their overall quality and variety. After that the quality drops quite a bit, though, and although almost all of the missions have something nice going for them (the shit-your-pants atmosphere of Return to the Cathedral, the ancient maps you use to find your way around the Old Quarter or The Lost City, the fact that Escape! at least manages to be properly fucked-up if nothing else), the game never quite picks up the same pace again, and the level design is in many places much weaker as well. I do like the last mission, so the game ends on a strong note, but the couple before that are not really something I ever look forward to when playing the game. Thief Gold is kind of a mixed bag in comparison to TDP, as Thieves Guild totally blows and ruins that flawless streak of early missions, but on the other hand I like the other two new missions quite a lot and think that they improve the mid-game considerably, even if The Mage Towers is a tad lengthy and formulaic.
The individual levels in The Metal Age don't generally stand out quite as clearly as the more varying ones of the first game, but their general quality is just really fucking good. Almost everything up to and including Life of the Party is either solid or outright brilliant, it's like a highlight after a highlight. After that the game loses its steam a bit and starts to drag a bit, though: Precious Cargo and Kidnap are just pretty anticlimactic after what you've gone through by that point, Casing the Joint and Masks obviously feel pretty lazy and unfinished, and Sabotage at Soulforge almost comes off as a chore. This is partly because the levels just aren't as great as the ones that came before, and partly because the game doesn't have the variety Thief 1, which just kept throwing curveballs at you and time after time took you out of your comfort zone. After so many huge missions you just may be a bit worn-out when you reach the endgame.
Neither game is perfect, both games are amazing, and together with the inferior (but still enjoyable) Deadly Shadows I consider Thief to be perhaps the best game trilogy ever made — the gameplay, the sound design, the art style, the writing, it all oozes class in a way that very few games have ever managed to reach. I naturally picked both because the poll allowed for it, but if I was forced to choose, I think I might go with The Metal Age because I feel it's the more consistent of the two, even though the first half of The Dark Project is still unmatched when it comes to stealth games, or maybe even games in general.