I liked Escape from Monkey Island - pretty graphics. A note for Vampire Naomi: The interface is similar to GF, but a more updated version of it.
With the exception of Fate of Atlantis, I was pretty familiar with Sierra games before I was familiar with LucasArts games.
Some favorites of mine that haven't already been mentioned:
- Laura Bow series (my favorite of all)
- King's Quest series
- Quest for Glory series
- Gabriel Knight series (all of them!)
- Nancy Drew series
- Titanic: Adventure Out of Time