While I would be quite annoyed, I could weather losing all my GOGs without any real pain. I very much enjoy them but they're not essential to my hobby. I can get by with Super Tux if have to. ;)

Sadly, none.

I don't get attached to my games too much. Starcraft 2 may be the only exception but that's because I play it with a friend and my brother and the whole social thing with them is what keeps me playing it. I would have stopped already if they weren't around to play it with.

I enjoy my games for as long as they last, but rarely play them again once I've beaten the campaign and played around with different characters or explored most areas. Then move on to the next one.

Good question. At the moment... 12 out of 30. There are more "essentials" on my wish list but haven't gotten them yet.

Out of my 47 games, there are only 2 that I would truly miss if I could never play them again: Arcanum and Septerra Core. Arcanum because it was truly enjoyable, and Septerra Core because I found it pretty fun to play, but also because I was intrigued by its atmosphere and story (I have yet to finish it).

That means I would only really miss 4% of my games.

Tbh, I have a LOT of games that I view as essential, either for originality, or because of the trends they kickstarted.

Another World: 15th Anniversary Edition
Fallout
Fallout 2
Disciples 1 & 2 and all expansions
HoMM 1-4
Arcanum
Master of Orion 2
Sid Meier's Colonization
Star Control
Star Control 2
Wing Commander Privateer
Betrayal at Krondor
Lords of the Realm 1 & 2
Lords of Magic: Why? Because despite the shit it got before the SE edition fixed the absolute bugginess, it was a great game. Why would I consider it essential? Specifically, because it represents both the best and worst parts of gaming in this era.

I'm not so attached to any of my things because they really are just things. I suppose I have a special place in my heart for most DOS games. The GOGs I feel are "historically significant" are;

The Zork series
The Ultima series
The King's and Space Quests
Fallout
Another World
Wing Commander III
The 7th Guest

"Essential" as in I am unwilling to part with them because I have played them so much that they have become a part of a past of good memories. Well of the roughly 210 games I own, 80% of which are DD, I will have to say none in particular. Like El_Caz, I don't get so much of an attachment in which I must have them in my collection besides having money spent on them.

If I have to choose something that is close to being "essentials" meaning games I've played and finished more than once or twice and still want to play in the future again, I would say:

- Fallout 1, 2, 3 and NV (must have played FO1 and 2 half a dozen times each since their releases in the 90's and played 3 and NV twice each too)
- The Sims 3 (well I guess this doesn't quite count, but... ^_^)

I think that's about it.

Edit: reading some of the other posts, if it's about having a special place in my heart, then they would have to be:

- Fallout series (minus Tactics)
- Dune series (the adventure game, 2, 2000)
- Baldurs Gate series
- Neverwinter Nights (first one)
- Kings Quest series
- Might & Magic series
- Quest for Glory series
- Master of Magic
- Civilizations 1
- Alpha Centauri
- Outpost 1 and 2 (they weren't that good, but they were memorable)
- Need for Speed (2, 3, 4)
- Screamer 1
- Wipeout XL

Ahh! The list is going to go on and on! ^_^

With you on this, almost every game has something unique and intriguing about it, I've been known to pick up a game simply because I know something odd about the development team or some such.

It's always sad to see the product of human endeavor be destroyed, I'd even be sad if Daikatana disappeared (not that I'd personally want to play it).