Technically, Legend of Grimrock but I have a DRM-free copy from humble so I do have a copy just not GOG.com. However, I don't own a copy of Real Myst.

Reason for edit: Too much technically the first time

Theme Hospital. I played enough of it when I got a physical copy(ages ago), so I haven't felt the need to purchase a digital copy.

The first game on the list I don't own in any form is Witcher 2, and I doubt I ever will, after playing the first game.

Hmm, Heroes 3 but I have it on CD.
The 1st one that I don't own anywhere would be Master of Orion 1+2.

P1: nothing
P2: Outcast 1.1
P3: Descent 3 + Mercenary
P4: Incredible Machine Mega Pack
P5: nothing
P6: Zork - Grand Inquisitor

Then things get a little less thorough. I'm missing four from Page 7!

Planescape: Torment. Oh, and Baldur's Gate 2 is the second one I don't own. I keep meaning to get them, but whenever I have money to spend, I look at them and figure, "These go on sale a lot, so I'll just wait until then to get them," and then when the sales come around I find that I have no money.

Enebias, we are talking about GOG's best-seller list. What is the first game that you don't own from there?

Nice idea zeffyr,
here its Dungeon Keeper2 and Theme Hospital - DK2 is excused i played it a lot when it came out
So maybe if my backlog is below 10 i may get it also at gog.
But for TH: I was never into the "themes" series. Really not sure why - maybe cause they look so time consuming to me.
In that time i can possibly do two or three games of the backlog ;-)

As many others, Dungeon Keeper 2, for the same reasons.

I don't own:
Baldur's Gate 2,
Baldur's Gate,
theme Hospital,
Neverwinter Night 2,
Dungeon Keeper 2,
Icewind Dale,
Temple of the Elemental Evil,
Master of Orion 1+2,
Rollercoaster Tycoo: Deluxe,
Dungeons and Dragons; Dragonshard,
Forgotten Realms: Demon Stone,
SIMCITY 200,
Freespace 2,
Rollercoaster Tycoon 2,
Ultima 7,
FTL,
Total Annihilation: Commander Pack,
Syndicate Plus,
Might and Magic 6,
Caesar 3,
Pharaoh + Cleopatra,
Wing Commander 3,
Wing Commander 1+2,
Wing Commander 4,


And for the rest of the first page I only own Deus Ex, it probably would have been easier to list only what I did own.

In order, first two pages:
Myst -- Because it always was a horrible game and nothing can change that.
Riven -- Same reason.
Leisure Suit Larry: Love for Sail! -- not interested.
Another LSL game.
Jagged Alliance 2 -- own elsewhere (Humble due to Kickstarter of newer game).
Real Myst -- see above.

Top 3 ranked bestsellers I own and regret:
Theme Hospital -- I just don't get? Even with CorsixTH... There's nothing to do in the game? It's just not fun? I generally like "Tycoon" type games. I bought it went it was so hyped and was like, "This is it?"
D&D Dragonshard -- I keep trying it again every couple years and it's still bad. It's just another also-tried RTS that, fortunately, ,tried some different mechanics.. that all failed miserably and just created a miserable experience. Got with the rest of the great D&D bundle.
Freespace2 -- The "move freely in all three dimensions" just is too foreign for my brain to wrap around. It was bundled with something at some point. I might give it another try again later.

See what happens when you are illiterate? :P
I though we were talking generally about famous best-sellers on GOG (hence why I mentioned Monkey Island), not famous best sellers on the list with the same name (then I added non-GOG ones just because)!
zeffyr wrote it clearly, though, so... my apologies! Too much beer, I guess! :P

So, let me answer properly: the first game I'm missing is Dungeon Keeper 2. I still have to play the first - gently gifted to me by GOG and EA during the last Valentine day- before considering a purchase, though.

On the same page, I don't own Theme Hospital, Neverwinter Nights 2 (I'm very interested in Mask of the Betrayer, but not as much for what concerns the rest), Heroes of M&M 3, Arcanum and many, many others.
The only one I'd really like to play is Freespace 2, but I always pass for a reason or another, mostly because I don't have a joystick.