Thief 1: TDP or not, up to you. Get Tfix.
Thief 2. Get Taffer patch
Some say Thief 3 but I didn't like it too much and ended up dropping it.
Splinter Cell series: SC, SC: Pandora Tomorrow, SC: Chaos Theory and SC: Double Agent for XBOX or PS2. Don't touch the ones after.
Metal Gear Solid series. 1-3 are great, 4 is a massive slog and is basically Kojima throwing a hissy fit in vidya form, PW sucks and V is great but half finished.
Deus Ex, HR and MD are good for stealth, the latter two being current year gaming.
Dishonored 1 and 2 but non-lethal stealth is boring as fuck in those games.
Dark Messiah is pretty good for stealth.
I plan to play Kingdom Come Deliverance again, with a focus on stealth, sneaking, assassin playstyle etc. don't know how good it actually is with its stealth though.
I'm not a Hitman fan, but many swear by the series possibly with the exception of the first.
Death to Spies, low budget and buggy WWII Hitman, haven't played but people say good things.
Tenchu series, didn't get very far with the first so can't say.
Mark of the Ninja, awesome 2D stealth game.
Styx 1 is great if limited with its black/white stealth system. 2 is casualize and Styx becomes an insufferable Deadpool ripoff.