Bioshock is definitely one I've been considering but the... uniqueness of the setting kinda turns me off.
The setting rather makes that particular game. IMO, BioShock is a magnificent game. The mechanics of the guns and plasmid use (and interesting combos), the oppressive and haunting setting and the twisting story intertwine to make one of the best games I've ever played.
And you can get it for pretty cheap now.
Fable III has been getting some really poor reviews
Depends on what reviews you are reading... Wikipedia's
article on the game lists a fair number of video game magazines/sites and their scores, and it largely places at 8/10 or above (IGN gave it a 6 on PC, while Gamespot gave 7.5 and 7 to 360 and PC respectively - everyone else gave it higher).
If you liked Fable 1 and 2, there's just more for you to like in Fable III. If you haven't played the previous games, Fable III makes (IMO) an engaging entry point. Lionhead keeps refining their combat and making it more fluid and intuitive. And Fable III has the really cool Spell Weaving feature, allowing you to combine spell effects and make some really awesome combos (Fire + Lightning = Zappy Burny Death).
And my own comments on a few other suggestions, based on my own play experience:
Elder Scrolls: I found the main story in Morrowind and Oblivion each to be largely forgettable. Oblivion's finale, however, was amazing! The most fun in Morrowind and Oblivion is basically hunting down the best toys and doing whatever you feel like. Dungeon-crawling, sniping
guards bandits, hunting down monsters (including vampires) or joining in a variety of guilds and completing their quests to eventually take over leadership and all the benefits they bring.
(side note: Rhun points out the Bloodmoon expansion to Morrowind. That has a very cool main quest, and the Tribunal expansion has a pretty good intrigue-based main quest. Game of the Year Edition of Morrowind is recommended so you don't have to purchase them separately.)
Assassin's Creed series: Excellent. Not perfect, but definitely approaching. So far, I like AC2 best overall.
AC1 sets up the story and introduces the mechanics (and the signature weapon of the games - the hidden blade), but I had a few issues with it... One being you cannot swim. At all. Fall in the water and you instantly drown, which made it very troublesome when one of your missions has you sneaking onto a boat in the port and the jump sensor is a little off. Another annoyance of mine with AC1 is that guardsmen are completely paranoid about you and you alone. A crazy guy comes up and shoves you, the guards get mad at you and will most likely draw their swords and attack.
The game's missions are repetitive... The real differences are just where and how you kill the target.
I was bored by the main character(s). Altair, to me, sounded like a bored idiot (largely, I blame the voice actor). The lack of interest in the character bothered me sometimes, but the rest of the game was enough to overcome it.
And the ending is absolute garbage, just setting the game up for a sequel.
Thankfully, it received one and AC2 was an improvement in pretty much everyone's opinion. Combat is better (actually being able to fight with your hidden blade(s) is very cool), guards are less paranoid, the free running is more fluid than before (and it was pretty good before), missions are a lot more varied, and the main character is interesting and demonstrates a fair amount of growth through the game.
AC: Brotherhood refined combat further and provided more mission variety (plus brought in the Recruits mechanic), but the fairly boring kills and lack of connection with most of the targets made for a lesser experience than AC2 (IMO).
You can also look at some downloadable titles, depending on what platforms you have. Speaking of which, if we know what consoles/platforms you have, we can better recommend games. The same goes for preferred genre. You point out Mass Effect, but that is pretty much a hybrid game unto itself.