For games from yesteryear it would have to be Master of Orion III.
First, it's the most buggy pile of crap ever to be rushed out the door before its time (its time was "never," but there was money to be made, so out it went!) Seriously, I played Fallout 2, unpatched for fuck's sake, and MoO3 was worse.
Think you can tell a planet to build something? Fuck you! A few turns later, you'll check back and see as soon as your back was turned the planetary governor -- who you had switched off -- has turned himself back on and set the planet to produce transports! Which might be okay, if you didn't already have fifty of the things from all the other planetary governors doing the same thing. Never mind that we're about to be attacked, and our survival as a species depends on us having combat ships to defend ourselves. Transports for everyone, huzzah!
Think space battle will be exciting, visually stimulating, or comprehensible? Think again! It's a grid with moving dots! Seriously, it's like something pulled out of a 1983 movie. In a game released in 2003. I kept expecting a shockingly young Matthew Broderick to pop up. And don't think you'll have any idea why you're winning or losing, or even what's happening. Simply inexcusable.
The worst part is you can see jagged shards of the game's shattered potential lying around. It's got MoO2, one of the best strategy games ever, as a predecessor, and you can see lots of little things that carried over. The layers-of-an-onion approach to management was a neat idea that should have allowed you to run things in broad strokes or drill down and micromanage to your heart's content, provided of course that all your choices didn't undo themselves as soon as you looked away.
For more recent games, I avoid the following labels.
Halo. I've played Halo, I found it mediocre. I don't understand the fuss. Please don't tell there is no such thing as Halo fanboys. I've met some.
Mass Effect. Yeah, that's right, I said Mass Effect. Heard good things, bought the first game, want my money back. Something is deeply wrong with this game. It crashes randomly even after patching. With all graphics on minimum my framerate doesn't get above 15, ever (on a machine that can run Crysis smooth as silk with good settings). Oh, and don't try to save, or the game will crash! This franchise isn't getting any more money from me.
Call of Duty. The single player campaigns are good, but too short. Multiplayer is why you buy these. I found the online experience a chaotic, selfish, twitchy mess of rag-doll physics and foul-mouthed twelve-year-olds. No strategy, no real tactics (quickscoping does NOT count), no teamwork. Online games are often known for unpleasant communitites, but I'd have to say CoD "wins" the trophy.