Sounds cool. Let me think:
(I'll try and do this in a particular order)

1. Xenoblade
2. Baten Kaitos: Eternal Wings and the Lost Ocean
3. Baten Kaitos Origins
4. Mount and Blade
5. Phantasy Star IV
6. Super Robot Taisen: OG: Endless Frontier
7. Paper Mario: The Thousand Year Door
8. Dragon's Dogma
9. Fallout 2
10. The Witcher

Well... that's sort of in an order. The first five are my favorites in many, many ways. The remainder could have been swapped out with a lot of other games, mostly because of how good they and the next ten or so are not that they don't deserve the spots they hold.

1) Ultima Underworld
2) Fallout 2
3) Deus Ex
4) Eye of the Beholder 2
5) Baldur's Gate 2
6) Ravenloft - Strahd's Possession
7) Knights of the Old Republic 2
8) Shadowrun (Sega Genesis)
9) Dark Souls
10) Gothic

PS: Do the games have to be available here?
PPS: I guess not.

My top 10

Morrowind
Planescape Torment
Fallout 2
The Witcher 3
Baldur's Gate 2
Icewind Dale
Star Wars KOTOR
Gothic 2
Ultima 7
Ultima 4

Damn this was harder than I thought. If you asked me again in 1 hour I would probably rate differently

1. Ultima 4 (nothing like your first one)
2. Vampire : Bloodlines (the full immersion experience)
3. Arcanum (the full possibilities of a game)
4. Vampire : redemption (the endearing experience)
5. Fallout 1 (so many approaches to one world)
6. Shadowrun returns experiences (the three new games in here, they are all the same glorious sixth world experience to me, my most loved setting)
7. Shadowrun (the nintendo game, no other game I've beaten so many times)
8. KOTOR (a sad star wars experience)
9. KOTOR 2 (a star wars philosophical experience)
10. Lost Odyssey (a litterary jrpg experience)

I've yet to experience so many other rpgs and that was one hell of a fight to select those ten ones. :)

My list. Morrowind is my all time favourite game :)

Morrowind
Baldur's Gate 2
Dragon Age: Origins
Divinity: Original Sin
Mass Effect
Might and Magic 6: The Mandate of Heaven
Might and Magic 7: For Blood and Honor
The Witcher
Skyrim
Divinity 2

(Neverwinter Nights, only played modules...)

I really like it. It felt like I was doing a table top dungeon crawl and I like the way they integrated a turnbased system without using a grid. Plus it's AWESOME that they were able to include utility spells and have them be real powerhouses (I'm looking at you Oil - or Slick - I forget the exact name - but take that all you DPS hounds!).

Of course, it holds the dubious distinction of being one of the only games I've ever played in a modded state. I didn't dare even fire the thing up until I knew I had Circle of 8 properly up and running.

1. The Witcher (yes, the first one)
2. Divine Divinity
3. Knights of the Old Republic
5. Fallout New Vegas
7. Drakensang
6. KotOR 2
7. Might and Magic X
8. Mass Effect
9. Deus Ex
10. Freedom Force

I'm not 100% sure about it though, especially the order. Some I can admit are better, but I like others more for totally subjective reasons like setting or art style or whatever.

If I made a list it would be too fanboy-ish so I better sit this one out.

Dragon age origins
Dragon age 2
Dragon age Inquisition

These are the games I had fun in, there are plenty others.. but word up for the Dragon Age franchise.

Love these games.

Dragon Age is here?

Oh! That's cool but for a second I got all excited. XD Thought I had missed a release or something. :P

In no order

1 Suikoden 1
2 Suikoden 2
3 Witcher 1
4 Witcher 2
5 Witcher 3
6 Vampire: Bloodlines
7 Under rail (Yes it just got released)
8 Gothic 2
9 MOUNT & BLADE: WARBAND
10 Shadowrun: Dragonfall

Ultima Underworld 2
Ultima Underworld
Exile 1/Avernum 1/Avernum: Escape from the Pit
Exile 2/Avernum 2/Avernum: Crystal Souls
Exile 3/Avernum 3
TES: Daggerfall
Ultima 7
Lands of Lore 2
Chrono Trigger
Icewind Dale

The OP has Diablo games in his list, so I'm assuming we're talking about RPGs in general.