You should try AC2, they did literally fix every single complaint they received from AC1, it's one of the best examples of a developer listening to criticism and using it constructively.  It was also one of the best games of the gen, in fact it was so good they keep trying to make the same game every year since then.

Probably Diablo 3 aka the Modern PC farming simulator.

As for games I was excited to play but completely underwhelmed by: Tomb Raider: Underworld, Dragon's Age

Games that could/should have been so much greater than they were: Wii Music / Animal Crossing City Folk - both of these games had the potential for 20m+ sales if Nintendo had done them correctly.

Bayonetta was directed by the guy who made Devil May Cry, which in turn inspired the creation of God of War... hence the similarities between the three titles.  So essentially you just said that Hideki Kamiya copycatted his own work.  Except DMC 1 didn't have Witch Time, Dodge offset, Torture Attacks, Executions, Wicked Weaves, 9 completely different weapons with full movesets, useable Angel Weapons, 100+ combos, optional accessories (all of which alter the gameplay in various ways), a survival arena, a hidden boss fight, etc.     

A demo wouldn't communicate the game's appeal well. It never really pretended to be, well, good, at least from a gameplay perspective, but then you never experience Pure White Glastonberry Extreme, Sir Henry Motherfucker, machine-gun-nipples, or Bad Girl.

Or, for that matter...

that doesnt make alot of sense, a demo is supposed to convince me to buy the game, if it doesnt do that its either A) not a very good game or B) they put together a crappy demo and ill tell you what, i wont spend money hoping its B) not when there are actually better games in that same genre out there. Hell you even said just now it wasnt good. I personally hate the idea of having shake my controller like im jerking off(mature) while fighting boring enemies with boring combat to get to the (ill admit) pretty cool bosses

GTA IV - The story was dull, the characters were snoozers. But most importantly the city was so lifeless and static. In past GTA games I could play for hours just going around doing stuff. Just enjoying the world. In GTA IV, not so much. I have enjoyed Saints Row for this reason much more this gen.

Halo 3 - Loved Halo 2. For some reason I just could not get into Halo 3. Hated the maps, hated the weapons. SP was boring.

CoD MW2 - The game was phenomenal for about a month, until it became apparently how terribly broken, unbalanced, and glitchy it was.

KZ2 - Bought a PS3 for this game. Ho hum by-the-book SP campaign, terrible controls and game design in MP ruined it for me.

Twisted Metal - Sony rushed this one out knowing it was broken and as a result, the community was gone by the time they fixed it. Which was a shame because the game is awesome when it works. Just rarely works.

GT5 - One of the rare occasions for me when presentation and menus can destroy a game. I have read that its much better and super improved but I have moved on. Its not fun when the menus are so terrible and take forever to load and navigate. On the gameplay end, I didn't like how only 1/5 of the game was actually next gen.

BF3 - I loved BFBC2 and I loved BF2. But BF3 is not Battlefield, it's a bad mix of BF and CoD.

Out of all these and others I am forgetting, I guess personally the answer would be MW2, because I spent literally hundreds of hours on MW1 and World at War each.

Damn it Khan stop posting that version of the song. You know it's inferior.

The update in GT5 didn't improve the menus or the load times for the most part it just improved the driving mechanics and added online single player events.  Even with the optional 8GB install the load times still are pretty long.

RAGE and Mass Effect 3. They're not awful games, but disappointing when you consider the previous works of the devs.

But THE most disappointing game for me would have to be Ninja Gaiden 3. Just terrible ...