i remember when i was a kid back in the day... my top 5 favs were:
(not counting the ones that i dont remember what they were called)
-jurassic park
-chip's challenge
-crystal caves
-museum madness
-ski free
~

If we are talking about back in the day and not today...

Magic Candle 2 & 3
System Shock
Warcraft 2
Xcom
Doom

I didn't really have a good computer or money for games back "in my day". Most of those games listed were played on my parents computer and mooching from my college friends. Now, with Abandonia, I get to mis-spend my youth all over again!

Hmm...for me, probably Gender Wars, Kingdom o' Magic, and Simon the Sorcerer 2: The Lion, the Wizard and the Wardrobe

Ok, for me, and almost all time favourites:

Loom: First adventure ever, beautiful, great story, and I studied music by that time.
Monkey Island I and II: Made me laugh for good, and every time I play them again I discover new jokes, even some I wasn't supposed to understand as a child. The third one was so dissapointing compared to those two...
Lands of Lore: Great fantasy atmosphere, great music, good puzzles and a nice and classic story.
Daggerfall: Complete freedom and almost eternally replayable.

Mmmm, I so love DosBox...

where to start :)

Countdown
Day of the tentacle sam n max
Wolfenstein
xenon
monkey island i & 2 ( I thought no 3 was also pretty good as well )
populous
zork
Jazz Jackrabbit ( + all the other appoge platformers :), geez just amazing how long did they take to d/l from the bbs and now less than a minute !)

all the usual suspects...

I could carry on for ages as it's really hard to narrow down my fave ones

Memory lane and not discovered later:
Keen series
L.A. Crackdown
Sopwith
Simpsons arcade
Beetlejuice Skeletons In The Closet
F-15 SE II
Wolfenstein 3D
MI 1/any quest title

Electro Man - why, I've picked it back up a few days ago!
Pinball Illusions
Space Hulk

These are more or less stable. There are others, but... well, their list changes frequently.

Hmm...my 5 favorite from back in the dos days? These aren't necessarily the best, but they are my top 5 as I remember them.

1. Ascendancy (Still my all time favorite game, i still play it every month at least once.)

2. Dark Force (I still play this one too. Best FPS game of the Doom era)

3. Mario is Missing! (Pretty simple game, but I have fond memories of trying forever to beat this game. It's long and tedious, but rewarding for some reason...)

4. Bioforge (Awsome game, amazing graphics, good sound, good story, a truly interactive movie)

5. Wing Commander 2

Honorable mentions: Doom, Rebel Assault, Raptor, Crusader No Remorse, Space Quest 3 & 4, Captain Comic, Where in the World is Carmen Sandiego Deluxe, F15 Strike Eagle 2

I love Max and Doom.

So far ...

Gabriel Knight: Sins of the Fathers
Eric The Unready
Secret of Monkey Island
Sam and Max
Indy Jones and the Fate of Atlantis (Lucasarts rocked)
Out of This World
Betrayal At Krondor
All the Space Quests (It was a dark day when the the series ended.)

Memory Lane? Wow. Anybody remember the Zork text adventure games?