I remember there already was a thread like this but it was a while ago, so why not. But as others have pointed out, it's not really informative, because the bottom of the list is dominated not by underrated or bad games, but by those that were only recently released here.

Anyway, not counting Edna & Harvey: The Breakout, it's Stick It To The Man for me, not a perfect but a really nice and enjoyable game that I've already played through. Soon it's probably going to be replaced by Bad Mojo.

If we count all games we own, even if we don't bought them from GOG, then it would be Depths of Peril. I got it in a bundle, I've played it for a bit and it's okay, but the concept is not something I'm really interested in.

Triple Town

Aint no shame!

Quite a few at the bottom of the barrel are on my wishlist, my slacker ass just keeps missing the sales :P

This is exactly what I just experienced when I tried to get to the end of the list. Another unrelated thing that sucks about the filters is if you try to find a specific publisher or developer you have to manually wade through page after page of developers in random (non-alphabetic) order with no way to shorted the list or find what you're looking for easily (reported to GOG long ago). Hopefully these problems are addressed in the next major site revamp that is supposed to happen soon.

Ultimate Body Blows, but in my defense, the nostalgia was the main reason. BB was out before there was a viable SF2 copy for PC that I could afford, so I played a LOT of crappy fighting games back in the day, including Brudal Baddle, Pray for Death, Rise of the Robots, Sango Fighter, right on down the line. Actually, I think crappy fighting games might be my guilty pleasure. I had FX Fighter as well. X_X

For my GOG catalogue, it's Risk of Rain.

For my non-GOG catalogue, it's Dustforce.

I've played neither; heard some good things about Risk of Rain and Dustforce was in a bundle.

It appears mine is BlazBlue Calamity Trigger. It's a fun 2D fighter and I personally enjoyed it more than a lot of the other fighters I've played. BlazBlue is a little buggy for some people though and I can understand that it wouldn't sell great for that reason. Personally, I had to lower the AA for performance reasons and have had no issues since.

Ok, I managed to finally get the whole list loaded but it took forever. :)

Ignoring the 12 GOG freebie games which all reside at the very bottom of the list, the next game higher than that which I actually own is "Dracula 4+5" of which I have not yet played. I want to get a copy of Dracula 1+2+3 and start the series at the beginning but it hasn't turned up in a sale since I've been watching for it yet.

Above that is "Apache Longbow", "Hind", "Sang-Froid: Tales of Werewolves", "Tzar: The Burden of the Crown", "Corsairs Gold", "Desperados 2: Cooper's Revenge", "Celtic Kings: Rage of War", "Gothic 3: Forsaken Gods Enhanced Edition" - none of which I've gotten a chance to install or play yet.

Then finally "Surgeon Simulator 2013" is the first game from the bottom of the list that I've actually installed and played and I thought it was hilarious albeit niche humour which caters to a certain mindset and would probably not be widely popular outside of the niche. I enjoyed what I played of it though and plan on digging into it again sometime when I'm bored and looking for a laugh although Goat Simulator has kind of taken my attention for that type of game for the time being. ;oP

Sensible Soccer 2006.
I guess the reviews already covered what I would say of the game...

Lowest non-Free - Edna & Harvey: The Breakout.
Lowest non-GOG - Risen 2 (Steam bundle)
Lowest retail - Bad Mojo Redux
Lowest wishlisted - 1954 alcatraz

Lowest worth noting (most likely NOT due to newness on GOG or lack of sales) - Dracula Trilogy. Really good atmospheric point and click. At the time, the graphics were amazing. Good story, nice cinematics, spooky audio, puzzles weren't over the top... highly recommend to adventure fans. Highlier recommend to adventure/horror fans. Highliest recommended to adventure/horror/Dracula fans ;)

Top 20 starting with the lowest seller:
01. DLC Imperial Edition - Upgrade (Divinity: Dragon Commander)
02. F-117A Nighthawk Stealth Fighter 2.0
03. Luftrausers
04. The Shivah
05. Dracula 4+5
06. American McGee's Grimm
07. Flatout 2
08. Humans Must Answer
09. Valdis Story: Abyssal City
10. Pixeljunk Shooter
11. Deadly Premonition
12. Cognition: An Erica Reed Thriller
13. Divinity: Dragon Commander
14. Heretic Kingdoms: The Inquisition
15. Shelter
16. Tomb Raider: The Angel of Darkness
17. Race the Sun
18. Aarklash: Legacy
19. Shadow Man
20. Shadowrun Returns

I notice people are posting their lowest non-GOG games also... but how do you determine the best-selling status of non-GOG games?

I based everything on how it was ranked by GOG. (others may be doing it differently)

Wow, surprised at some of the low sellers. I plan to get Silent Service which is way down there.

My lowest I actually bought so far : Lowest first...

Greatest Battles
Thunderscape
Strategic command
Alien Shooter + Expansions
Combat Mission: Beyond Overlord (my favorite wargame on GOG)
Catacombs Pack
Manhole: Masterpiece Edition, The (got for the grandkids and never used of course)
Avadon 2: The Corruption
Arma Gold Edition
Waxworks

That's my list of ten. I enjoy Greatest battles and Love!, Combat Mission. Strategic Command so-so. The others I have not gotten to yet. Just got Waxworks and Catacombs on sale. I mostly just wait for the sales now. Unless it's a real classic like Pirates!, Bard's Tale, Wasteland etc.

Ignoring both the Edna and Harveys and the re-release of Strike Suit Zero...

For games owned anywhere, Beatbuddy is the lowest, Kentucky Route Zero the lowest I've played.

For games owned on GOG, The Adventures of Shuggy is lowest, Valdis Story the lowest I've played (a bit; it's not bad, but the constant health loss due to cold in the ice area is annoying, and hunting for means of countering it feels like work).

FX fighter was great!