I'll give it a go too. I never really made a top list so will see what becomes of this.
1. Discworld Noir - This is easy, it just is the best. The great mystery, the logical puzzles, wonderful characters, perfect humour. Enjoyable piece from beginning to end.
2. The Longest Journey - Epic adventure, can't have enough of them. And April Ryan, who doesn't love April Ryan? Great mix of cyberpunk and fantasy.
3. Gabriel Knight - The Sins of Father - The original Gabriel Knight always grabbed me unlike the sequels. Wonderful obnoxious character(s), interesting voodoo mystery. Jane Jensen's super writing is just that - super.
4. Gray Matter - Yeah, I'm serious. It might be that it will drop a bit whe time fades the memories etc. but I just really loved this one. Shot up right over the GK sequels and a bunch of other great games too. Story, characters, atmosphere it's all there again and I really like how it somehow manages to be a detective kind of game without either of the main characters being anything close to a detective.
5. Still Life - Another super detective story. Cut a bit too short but still brilliant. Too bad they sort of ruined the possibility of real ending with the subpar sequel.
6. Dreamfall - Super story, would be higher up if it wasn't for the clumsy controls and silly action scenes. Truly moving. I really hope they can get to Dreamfall Chapters soon.
7. Grim Fandango - Like Noir it's set up in a wonderful world that is humorous but not completely silly. A great adventure, engaging story with lovable characters and the right kind of humour.
8. Day of the Tentacle - Nowadays I am not so much into comic adventures but this one is a classic. It's just brilliant and I would love a remake. First time playing multiple characters (no, I never played Maniac Mansion, sorry) and solving puzzles "together" with them. It's super whacky, it's absurd and it's actually funny and not just trying to be.
9. Sam & Max - Hit the Road - Another whacky one that has stood the test of time. The absurd puzzles are still logical in the Sam & Max universe (and way more logical than most recent games anyway) and the story is grand.
10. Culpa Innata - This one kind of snuck up to me, I thought it was going to be another average crap with crappy graphics but it turned out to be very engaging adventure in an immersive cyberpunk world. I hope the sequel will see the light of day.
Honestly I also love Monkey Island trilogy (the 4th one was crap and haven't played Tales) but I couldn't find a place for them so they get a honorary mention or something
.
These all are 4,5-5 star games, there's a few more 4,5 ones and maaany 4 star ones that are great, but this is how I get my 10 when feeling it a bit just now.
