/vr/ - Retro Games

For Sierra, I'd say:
King's Quest 5 or 6
Police Quest
Gabriel Knight: Sins of the Fathers

Police Quest is pretty unique game in that its puzzles are designed to make you follow strict police procedure when dealing with characters/criminals (you'd better not use unwarranted force you sniveling cadet!).

Hon. mention to Space Quest 4, which is a funny space pulp adventure but is mechanically almost identical to King's Quest.

Also hon. mention to Phantasmagoria, but that's more of a "play around Halloween type of game." Not really scary, but atmospheric with some goofy FMV acting. The puzzles are pretty simple but it's worth it to take in the town. A superior FMV-style game is Gabriel Knight 2.

It's easy to burn out on these type of games, so I think it's best to look into a few, see which style you like the most, and play that one to completion.