Yeah that is the problem with pretty much all stealth games really ... but maybe it is not a problem at all, but rather just a effective strategy. I would like to see more permanent consequences for being caught in a stealth game. This should be done in such a way, however, that the player doesn't become frustrated and decides to simply go back to save point or restart a level.

Maybe, you could have guards take unpredictable routes when they have been alerted once ... in pretty much all stealth games guards never usually follow you into vents/tough to reach spaces, but maybe there is a chance (as in they will not always do it) they will explore vents or flood the vents with gas or something when they have gone to high alert. Maybe they switch to heavier weapons when a threat after one of their buddies is taken out. You could have guards lockdown certain doors or sections of a level which had offered a short cut to a goal prior to the lockdown. These are just some ideas, but, like I said, they need to be implemented in such a way that they add value to the experience rather than simply make the whole thing frustrating and annoying.

Getting back to Deus Ex HR, the AI may not have been perfect, but they aren't total idiots either. Their ability to see/detect you felt realistic, and they are very good shots with high damage weapons that down you in a few hits on the highest difficulty (which made getting caught something you really didn't want to happen ... kind of like in the Thief series).

Like you said, future stealth games (and games in general) need to find a way around the slow and steady approach of the player where you can just pick off otherwise difficult enemies one by one as their buddies get alerted for a few minutes and then proceed to go on like nothing happened rinse/repeat. You can do this in most stealth games and even most FPS campaigns.

Man f zero Gx was so awesome, hope they make a sequel

I remember that the original Brother In Arms struck me with a very reliable AI. People protected themselves. They used combat techniques. They understood danger. You could give orders and the soldiers wouldn't carry them if they were suicide orders, just like charging an stationary machine-gun. It was a long time since I last played it, but I think it was brilliant for a Xbox/ps2 game.

Pure Chess on PSN.

Really? It was almost as bad as Assassin's Creed to me.

For me it was The Jackals in Halo: Reach. They were so dang sneaky and kept moving out of my range. Then if I move, they would sneak down this hill. It's towards the end of the game, but I felt almost like there were people controlling it. Everything I did the responded in a way that I had trouble hitting them. They would sneak out of range, move when I had them on a sniper - just as I am about to shoot.

Them, not so much. They didn't have trouble hitting me.

The first Halo on legendary difficulty was mind blowing at the time.

I'd agree with this. FEAR and Half Life are really the only games where the AI has stuck out.

Having said that, the AI in Far Cry 1 was exceptionally "good", but to the point of being unrealistic and destroying immersion. They didn't appear intelligent, they just had superhuman senses normally reserved for the Marvel universe, lol.

This gen, Killzone 2 & 3 weren't bad, Last of Us was fairly good, but otherwise nothing really sticks out. 

Another example was the cmobat AI from Skyward Sword, they actually reacted to your movements really good.

I just remember it was good for its time.