SS1. I am on record as saying that SS2 is massively overrated, and I stand by that.
fuck you
The first few levels of SS2 when you are sneaking around, barely surviving, are great. However, after the game's twist, there's zero reason to keep playing. It turns into an awful corridor shooter with bullet sponge enemies, limited ammo, and constant retracing your steps over the game's maps to accomplish what more or less amounts to just a 1990s style FPS "find the red key for the red door, find the blue key for the blue door, and etc." objective list.
The Dark Engine is a wonderful, atmospheric, stealth engine, as Thief 1 and Thief 2 demonstrated. However, it's a shitty, FPS, combat engine, as SS2 and parts of Thief 1 demonstrated. Combat is floaty and lame with bullet spongey enemies who don't react to bullets or being hit. Thief 1 had brief enough combat sequences to still be a classic game. However, SS2 turns to shit once the game's focus shifts from stealth to combat.
And hitboxes in SS2 are completely fucked. I recall aiming my wrench at an enemy's head and swinging repeatedly, watching the wrench just pass through its head, doing zero damage, until I repositioned myself slightly, only to have the wrench suddenly connect, with no visual indicator as to why. It's a solid 2.5 out of 5 star game, which isn't totally awful, but it's nowhere near deserving of the hype it gets.
SS1 is better in almost every way, especially when it comes to making the most of the limitations of the tech of its time.