Picking just 5 from over the last almost 30 years I have been gaming is pretty damn hard.
King's Quest IV - First one with new Tandy system, always a special place in my heart for it
Leisure Suit Larry 3 - I still think this is the best out of the bunch (get all 5 questions right at beginning, then use binoculars
)
Mass Effect - If picking just one then it has to be the first one
Civilization - I spent an insane amount of time on the first one and the second, third, fourth, and now fifth one.
Gabriel Knight: Sins of the Father - Really one of the best games ever made. Played it through countless times.
Now to be a little more general, I would have to just name series like:
King's Quest
Police Quest
Quest for Glory
Leisure Suit Larry
Conquests of Camelot and Longbow
Mass Effect trilogy
Half-Life
Civilizations 1-5
Witcher series
I'm sure there are many more but those are the stand outs for me. Of course as you may be able to tell from my list I was a huge fan of Sierra back in its hay day.