I've pretty much given up on any sort of gaming, but Broken Sword is the series that always drags me back in. A mere $15 investment within the next two days not only pays to make the game ever more awesome but also secures you a cheap pre-order download of the game.
http://www.kickstarter.com/projects/165500047/broken-sword-the-serpents-curse-adventure
I remember playing the original game way back when with my younger brother. We had just gotten our first Windows computer, a pentium 1, 133Mhz. :) He got more hooked on it than I did and played a sequel too. I prefered the monkey island series that I had been playing since I first got my amiga 500.
Looking back I think police quest and larry did more for my english skills than the swedish education system ever did. Maybe that's why I still speak and type in short concise sentances? :P
Best adventure game of all times is of course Grim Fandango that I still fire up once in a while to play it through. Viva la revolution!
I liked the Broken Sword games though the first three all had utterly ridiculous endings IIRC. Haven't played the later sequels.
Loved the original, could never get into the latter ones so much