Like many others it was a choice between SI and Bloodmoon But I still have to go for SI...
The landscape of Bloodmoon is cool (especially compared to MW) but it gets bored pretty quick SI is just beautifull and the story is really cool (and mind-twisting) and you have alot of choices to make within the game due the constant rivalry between Mania and Dementia... Plus the SI quests are alot more exicting then the Bloodmoon quests, wich (especially when you're in the Skaal village) get pretty boring....
So alot of reasons to choice SI...
but Bloodmoon has a solid second place...
I can't really call KotN an expansion (although it contains one of my favorite questlines) I always view it a bit more as an extension of the Oblivion MQ