Hmm, this is pretty hard to pick since I have more than 5 all-time
favourites that are very closely tied, and I'm not entirely sure about the "best" ranking among them since they are fairly different and fun in their own way. But if I had to pick 5 it'd be something like:
1. Planescape: Torment
- Planescape: Torment
- Baldur's Gate II:Shadows of Amn (+ Throne of Bhaal expansion)
- Vampire the Masquerade: Bloodlines
- Deus Ex
- Sid Meier's Alpha Centauri (+ Alien Crossfire expansion)
Story heavy role playing game in the very weird Planescape D&D setting, with an equally weird protagonist. While a bit hard to get into initially once the game gets going it'll be loads of fun. Different storyline than what most games tend to have, a quite weird protagonist whose choices thoughout the game does matter. Interesting set of followers as well to pick up along the way who differs from what you usually find in RPGs: a tiefling, a floating skull, a githzerai, a walking suit of armor, a fallen succubus, a modron and a burning insane wizard.
Very emotional at times, even though much of the story comes in text form. A real classic whose perfection is only marred by the anticlimactic ending, IMO.
2. Baldur's Gate II:Shadows of Amn + Throne of Bhaal
THE computer role playing game for PC, this huge game took me on average 250 hours to play through when doing all the quest. Set on the Sword Coast/Faerun/Forgotten Realms it's an AD&D game with Bioware's traditional realtime turnbased play style. Continues and concludes the story started in Baldur's Gate I the games provide the longest continuing storyline I've seen in any games. Interesting followers and antagonists, interesting places to visit, loads of quests, side quests and plots. The only disadvantage is that it's such a time sink to play since it's so long.
3. Vampire the Masquerade: Bloodlines
In RPG's you are usually some heroic character killing nasty undead monsters, so it's a fun variation to play "the other side" in this game, being put in the role of a character who's killed in the game's intro and turned into a vampire. Despite this you are given some freedom in how to act and interact in the world, allowing you to play a quite "good" and sympathetic vampire or a true monster, as you like. The game's "Earth, 2004" setting is also some nice variation from the traditional history/future settings RPGs tend to play out in. Lots of dialog, various interesting side quest and more emphasis on story and less on looting and killing for XP. Varied gameplay depending on what clan you pick for your character. Playing as a Malkavian girl is hilarious.
4. Deus Ex
Classic RPG/FPS hybrid which is strongly story driven but at the same time encourages exploration with its open areas with various optional side quests/missions/tasks sprinkled throughout them whose completion can have consequences later in the game depending on your choices. Amazing attention to detail in the game world where even the seemingly most irrelevant backalley bum has unique dialog and may offer you useful info if you are persistent enough. Near-future Sci-Fi setting.
5. Sid Meier's Alpha Centauri
Turn based strategy game from the team behind Civilization, the original genre-defining time sink game.
Science fiction setting where mankind escapes a failing Earth to survive and colonize Chiron, a planet in the Alpha Centauri system. The mission fractures early and the colonists split into various factions based on ideology, where each faction has its strengths and weaknesses, and the leader its distinct personality. Fairly dark mood to the game, since humanity's future can be fairly bleak depending on what faction you chose to play. AC was pretty advanced for its time with 3d terrain where elevation mattered, unit design workshop where you assemble your own units from the tech you have, etc, and a fairly interesting and vast research tree of imagined technological advances. Like Civilization it's a tremendous time sink and I don't dare count how many hours I've wasted on this game.