Similar to GastonArg, I cannot quite choose my top 5 games. Favourite (unnumbered), sure, but I find it very dificult to order them.
Baldur's Gate
Baldur's Gate II
Icewind Dale
Icewind Dale II
Planescape: Torment
...that's five of the top ones, not neccesarily the five topest ones... continuing...
Elder Scrolls: Oblivion (yeah, I started the series with this one and had difficulty getting into Morrowind afterwards, bite me)
Elser Scrolls: Skyrim
Fallout: New Vegas
The Witcher
Command & Conquer: Red Alert 2
Command & Concuer Generals
Broken Sword: Shadow of the Templars
Broken Sword 2: The Smoking Mirror
Broken Sword 3: The Sleeping Dragon (no, I didn't mind the 3D sliding puzzles, also I didn't yet play the later games enough to have an opinion on them)
Half-Life
Mass Effect
Mass Effect 2
Dragon Age: Origins
The Longest Journey (again, haven't played Dreamfall enough to form an opinion)
... as I said, no internal order.