Both games start to decline a bit towards the end, to be honest. In my opinion TDP peaks fairly early on while having some definite highlights towards the end as well, whereas TMA starts off with a couple of pretty good levels and then continuously raises the stakes until reaching its peak sometime after the halfway point of the game. TDP has more variety and weirdness towards the end, which unfortunately doesn't always translate to great level design and might put some people off, whereas TMA kind of offers more of the same as before but not just as good, which might make the last few missions a bit of a slog. Thief Gold does make the first game's pacing a bit better by fleshing out the somewhat weak McGuffin hunt of the middle part of the game with a couple of really good missions, although it also includes Thieves' Guild, which is probably the worst level in any of the three Thief games and feels more like a fan mission than most actual fan missions do.
I guess the first game is just a bit more polarizing, with different people liking different parts and aspects of it. For me the first five missions of The Dark Project are damn close to perfect, all completely different yet simply superb, and the rest of the game only comes close to that momentarily, like with some parts of Return to the Cathedral. Someone else might absolutely love The Lost City, which I've always found a bit of a bore, or even the insanity of Escape!, which to me is one annoying level. On the other hand there's probably a much greater consensus about the highs and lows of The Metal Age, with everyone loving the bank or Life of the Party while not being all that enthusiastic about what comes after that.
Personally, I consider both games roughly equal. I used to prefer Thief 2, as I played it before Thief Gold, and initially disliked the undead levels in Thief 1, but Thief 1/Gold grew on me after replaying it, and now... eh, I'm not sure if I like it more than Thief 2. They're both excellent games, and their levels are so vastly different in design that making such a direct comparison is impossible. It all comes down to personal preference.
Thief 2 takes longer to develop its atmosphere than Thief 1/Gold does, and overall the atmosphere isn't as thick. It has a stronger middle to late game though.
Thief 1 is really the best in the beginning. Cragscleft, Bonehoard, The Sword, Lost City, and in Gold you get Song of the Caverns. They're all really damn lovely missions, and the atmosphere is so amazing. Really, for me Cragscleft is THE mission that captures the Hammerites best, to me it feels even more Hammerite-y than the actual Hammer church you visit later, as it always seemed a little larger to me, and more martial. I guess it also was kind of a letdown that the Hammer church in Undercover is comparatively small to the cathedral you've glimpsed in The Haunted Cathedral.
Anyway, Thief 1/Gold has so many diverse and strong levels with an atmosphere so thick, it sucks you right in. It becomes significantly weaker when you return with the eye, as Escape! is one of my least favourite levels, Bedfellows isn't that great either, but at least the Maw was pretty fun and interesting, with cool twisted architecture and shit. Overall, the last few missions are underwhelming compared to masterpieces like Cragscleft, Bonehoard, Constantine's mansion. Those three are the holy trinity of Thief 1 genius level design.
Thief 2 takes much longer to get to its great missions. To be honest, I have much less vivid memories of T2 missions than of T1 missions, and sometimes I even forget some of the more unremarkable ones. I have played so many fan missions set in mansions that standard mansion missions grow a little muddy in my memory. First mission was nothing special, definitely less interesting than Bafford's huge place. Second mission was a great piece of level design, probably one of the longest and most non-linear of all the original Thief missions, but I find warehouse districts to be visually boring. Level design there is top though. Third mission... here is where it gets muddy and I remember much less about the order in which the missions happened. There was Ambush at some point, the police station at another, then you'd trail the courier...
Where Thief 2 shines is the mid-late game. I loved breaking into the bank, and Life of the Party is the mission that was so great it spawned its own "subgenre" among fan mission authors - the thieves' highway. I actually enjoyed Gervasius' manor, even though I had to break into it twice.
Thief 1 had more variety, it was a little rough around the edges sometimes, but it had so many absolutely great missions with unique themes and atmospheres. Thief 2 was less varied, with a lot of the missions being basic variations of "break into a building that has guards in it and steal something from there", so they're less distinctive, and only the best of the missions still stick to my mind, unlike Thief 1 where all of them do. Both games are excellent and it's unlikely we're ever going to see level design of their calibre in a mainstream commercial production ever again. The Bonehoard? The Sword? Life of the Party? First City Bank and Trust? Missions like that just aren't "marketable" anymore in this day and age, huge sprawling places where the player is given a handful of objectives and then left to his own devices. These are the kinds of missions that are essentially the "essence" of Thief.
