4. Zac McKracken 2

Zak Mc Kraken 2 Adventure Game
First, there was Maniac Mansion, then there was Zak McKracken. Those have been the first two adventures by Lucasarts that used the SCUMM technology and a point and click interface which revolutionized the adventure genre. Zak McKracken 2 is a fan project that looks and feels like the orginal.
It has everything that most adventures of today are missing. Funny dialogs, great interface that is easy to use and riddles that feel like riddles. Remember when you found a chainsaw in Maniac Mansion but no fuel and Fuel in Zak McKracken but no chainsaw ? Its the same type of humor, definitely recommended.