The Hitman games.
The first hand-full being the best... I'm not sure I'd recommend the latest couple...
Chronicles of Riddick.
Assault on Dark Athena has both the original game plus what many consider to be an expansion (instead of an actual "new game") and the original game is remade with the new game's engine and assets so it's much prettier to look at!
Splinter Cell.
First game up through Chaos Theory.
Deus Ex.
The original. Though stealth isn't a necessity or even an obvious option here, it supports the play style exceptionally well. I'd rank it up near the Thief series itself.
There are also a whole lotta mods and updates available online for it to try soften the pain of playing with the engine and assets being what they are (i.e. "old and ugly").
...
But.
The absolute pinnacle of Stealth is the Thief series (excluding game circa 2014): Thief Gold; Thief II: The Metal Age; and Thief: Deadly Shadows.
BEFORE you start to play though, go to the Through The Looking Glass site (ttlg.com) and be sure to download the latest TFix patch for Thief Gold; the latest TafferPatcher for Thief 2. Both of these have updates from as recent as January 2013 that make a SIGNIFICANT difference to the games - runtime fixes; stability on modern systems; texture and asset resolution improvements / replacements; widescreen gaming activation; etc.
There's a similar patch set available for Thief 3 too.
And in addition there are SLEWS more mods and updates that improve the graphics, skybox, gameplay, etc. though those mentioned just above are CRUCIAL for playing these games properly these days!
Just browse around, read some forum threads, etc. - there's SO MUCH! And they're pretty much current!
If you really, really struggle with the dated state of the games to the extent you find you can't play Thief though (and even if you don't struggle!) then the very next best option in my not so humble opinion (or the opinion of MANY others) is "The Dark Mod" at thedarkmod.com
It used to be a total-conversion mod for Doom 3 but is now a standalone game, absolutely 100% free for download, install, and use.
It's essentially Thief and Thief 2 remade in a modern engine by some of the most devoted and talented fans in the community.
Right now it has around 80 different missions available for it.
For around 2GB download, plus another 1GB for mission-downloads, I CANNOT RECOMMEND THIS GAME ENOUGH!!! It absolutely ROCKS!!!
The Dark Mod is what Thief 2014 should have been - it beats the AAA effort soundly and completely!!!