OK - here goes…
1. Baldurs Gate II
Honourable mentions to the rest of the series, which I still play through on a regular basis. The characters, locations, and imagination of the sidequests keeps me coming back for more. It captured the feel of P&P for me in a way that more rule focussed turn based games strangely often miss.
2. Fallout I
Simply awesome storytelling, great atmosphere, and loads of scope for different approaches. I prefer it to the second one… and I even like the time limit
3. Pool of Radiance
Probably a certain element of nostalgia goggles, but I think this is still a great game. The Gold Box games getting released on GOG made me very happy, although some of the design carried over from early D&D rules (eg racial level caps) are a bit annoying if you play through the whole series.
4. Deus Ex
The way the game allowed you to approach challenges in different ways was brilliant. The first couple of times I hacked my way into something that looked off limits to find story information blew my mind. I haven't replayed it in years though… Maybe time to fire it up…
5. Dark Souls
I'm not normally a fan of ARPGs, and it took me a couple of attempts to get into it, but when I did I was hooked. The level design is brilliant, especially the interconnectivity of the areas - something later games in the series didn't do as well in my opinion. The exploration was great.
6. Mass Effect 2
Planet scanning is terrible. Levelling up feels pointless. The individual quests are often too linear. Hacking mini games are annoying. But as an overall game I think it rocks. Great characters and an interesting story, totally scratched my "wannabe Captain Kirk" itch, and an interesting world to explore.
7. Divinity Original Sin
Colourful, fun, great combat. My favorite of the more recent rpgs. I even liked the humour, which puts me in a minority of one I think!
8. Dragon Age: Origins
The origin stories were a great idea, its a shame subsequent Bioware games haven't developed this idea further. Also, great choice and consequence, and an interesting human villain. The Landsmeet is a brilliantly written scene. I actually started thinking I was going to lose that argument… I was more convinced by Loghain!
9. Avernum: Escape from the Pit
I wasn't sure about including this one, as I only played it recently. But I had such a good time exploring the caves and working out how to beat the great quests that I think it's on the list.
10. Icewind Dale
Dungeon crawling in the Infinity Engine? Excellent! And some of the battles were so eye watering frustrating (looking at you, giant snake thing) that they remain scarred into my brain forever I think. Still, massively satisfying when they went down!
Further honourable mentions to the Grimrock series, Neverwinter Nights, and the Elder Scrolls series (particularly Oblivion, despite its many, many flaws).