Severance Blade of Darkness, I loved this game at release and we had an awesome community built around it, but playing it without rose tinted specs, I wish I had left it in the fond memories section of my mind, nowadays it just feels clunky....but, back in the day....

At least it did reunite a few of us from back on the old severance forums, so that was nice :)

Hm, I couldn't single out just one game in each category, but these games

made me a very happy customers indeed:
Arx Fatalis
Enclave
Blade of Darkness
Waking Mars

(Psychonauts and Beyond Good & Evil would have been among them, too, if I hadn't won them; but I liked them so much that I bought and gifted another copy of each)

made me regret buying them for the price I paid:
The Real Texas
Omikron: The Nomad Soul
Demon Stone (not that I expected much, but it still managed to disappoint me)
King's Bounty: The Legend (but mostly because combat is painfully slow on my rig)

(Richard & Alice would have been among them, too, if it hadn't been dirt cheap anyway. Still not really worth it, IMO. Maybe also Snapshot, but only because it appeared in a PWYW bundle soon after, before I was even able to run it on my PC ...)

I thought I would regret buying and ended up loving:
Enclave and Blade of Darkness again, and maybe Dark Fall: The Journal

were a gift that I was sceptical about but then actually enjoyed
Symphony
Guilty Gear X2 Reloaded

Regret:
Sanitarium - fun, but the game crashes on a regular basis. Mostly after a cutscene or, in rare cases, before.
Alone in thze Dark 2 - I'm well aware, thatb the games are old and that AitD is included for free. While I did enjoy the first, the second was unbearable. The game mechanics are not made for an action-game and yet, you're forced to fight your way through the game. and all Mooks have ranged attacks!
Harvey's New Eyes & Deponia - Technical issues. Whiley Eyes worked well until I hit Act 3, both games start to "break" after a few minutes of gameplay. The backgrounds vanish into a white void, so are animations replaced by white blocks. And teh same happens with "A new beginning", but not that frequently. Harvey and depo are much worse.
Zafehouse Diaries - I thought I could get into this kind of game. But I can't.

Favourite
Cat Lady - Hoo buy, what an atmosphere, what a storyline!
Heroes of might and magic 3 - Owned it years ago, but lost the copy. And now, I even have all expansions. And I even won a few campaigns without cheating.
Defender's Quest - Didn't know what to expect and bought it from the last "Pick 5"-Promo, because I had to pick one more. This was the most appealing and I do enjoy it a lot.

You missed the part where I said I feel like I mugged GOG :P I used KoDP as an example of a GOOD purchase, not a bad one. It's one of the best games I've ever played, you should give it a shot.

I suppose I probably ought to regret all the impulse buys (that were on sale at the time, or some other flimsy justification) that I've never actually played, but meh... ;)

I think my biggest regret is Heroes Of Annihilated Empires. Sounded right up my street - blend of RTS and RPG, story by a professional novelist, decent-sounding magic system etc. But... Damn! What I found was the cheesiest generic fantasy bollocks (seriously, the cut-scenes are APPALLING) and a ridiculous first level that for some reason I just could not complete. And I think I paid full price for it.

Favourite purchase is tougher. In terms of what I play the most it's likely to be the Infinity Engine AD&D games, or Neverwinter Nights - but then I already owned disc copies of a lot of them and knew what I was getting.

I was actually very pleasantly surprised by how much fun Freedom Force Versus The Third Reich turned out to be.

Regret FTL.
Most pleasant surprise is definitely Sacrifice.

My regret is Omerta. Not because of the gameplay, with that I am okay. I am just annoyed that I bought the DLC (at a price I am happy with), only for it to be bundled a few weeks later into the main game anyway.

My best buy here was Hotline Miami. I really enjoyed the game, and even managed to play it to the end after I figured out the control system and stopped running into walls all the time.

I impulsively bought and loved The Cat Lady. Everyone was saying how good the game was, so I picked it up in a sale and just could not pull myself away all day that I played it.

I was gifted Fez which I really struggle to get into. The gameplay is fun and I am sure I really love what is deep down, but the world itself just does not hold my attention.

Best purchase:
Probably Plainscape: Torment - a very good game with all the mods.

Biggest regret:
King of Dragon Pass: Great reviews, but it doesn't run well because of the fixed resolution and ancient unsupported engine, and there's a superior iOS version I didn't know about at time of purchase.

It is hard to tell which is the best buy for me on GOG - for many of the games i bought here i knew beforehand - so it is not kinda fair to mention them here - but from the ones i bought without knowing anything about them except reading reviews or looking at the screenshot - i was most pleasantly surprised with Beyond Good and Evil and The Night of the Rabbit
It is not easy to pinpoint the wors buy also - because there were not many complete misses - but i would name Jack Orlando as one - not because the game is That bad - but i had hopes for much more - interesting story, time setting, nice animation - but...

regret: recently? Shelter - that game has no save option and it isn't not even slightly as fun to play as I thought it would... I'd throw it back right this second if I could

best buy: recently? The Cat Lady - it's so awesome I barely believed how awesome a game can be :D

Aww, I was looking forward to that game. You mean it's basically a roguelike if you can't save?

No, the game consists of 5 or 6 chapters, and your progress is automatically saved at the end of each chapter.

Thanks for the info.

as SCPM says... but I never got far enough to cross to another chapter.

but maybe it's just me - I work very badly under time pressure, and this game does just that: the kids are constantly dying so basically you need to run forward (there are more ways to go so it doesn't seem linear, but you just pick one way and rush forward that way), trying to spot any food/animal to feed them... there's no time to look around, explore the surroundings or just enjoy the scenery

yes, I'd be a terrible badger mom :(

Biggest regret: Well, I don't have any major regrets because I tend to not buy crap, and instead research beforehand, but I guess Wing Commander 1+2 is a small regret in the sense that there's no easy way to customize controls and I think the default scheme isn't great. I also think PC WC feels a bit more stiff than the SNES WC. Lionheart is a semi-regret: I didn't play it very far because I just didn't find it that immersive. The only reason I bought it for was the perk system borrowed from Fallout: well, I saw it in action and that's pretty much what I wanted in the first place. I...guess those two are it. I don't regret any of my other purchases (even a little).

Favorites: Well, I'm not gonna name games I had played in the past because obviously I knew they were going to be good still. But games I've played only after getting them from GOG and liked them a lot would definitely include the Icewind Dales. Of the newer titles I've purchased, I would definitely praise FTL.