For DA:O
Warden's Keep (gives you unlimited storage and potential to the best sword in the game, plotwise it just gives backstory)
Stone Golem (best helmet in the game, cool party member no real plot)
Lost at Ostagar (neat, but kinda boring and disappointing)
Leliana's Song (backstory)
Golems at Amgarrak (hard no real story)
Morrigan's blablabla (endgame stuff, can be important storywise)
And then there's also Darkspawn Chronicles but that one sucks, and if you count Awakenings as DLC (it's an expansion pack) I'd get that cause well it's an expansion.