Zak McKracken and the Alien Mindbenders

The Good
When I was around 12 and this game first made a splash on the scene, I pretty much became obsessed with it. It was nothing I've experienced before on my trusty C64: its breadth and atmosphere felt just surreal. I could submerge in it for entire weekends. Granted, I used to progress awfully slowly in adventure games, prone to savour and take in every new location for quite a while.
On a less personal note, this game was the first truly accomplished, epic point-n-click adventure game -- or 'arcade adventure' as we called them back then. It really gave Sierra a run for their money by taking to the next level everything the then-simplistic Quest franchise stood for.
It had heartfelt dialogs, an intricate if goofy plot, plenty of good ideas and in-jokes, loads of locations, and even a catchy theme song. In other words, it was a cut above the Quest series it was designed to challenge.
Significantly, it also directly paved the way for the more sophisticated Monkey Island duo by boldly experimenting with the SCUMM design system.

The Bad
The maze scenes; these were obvious fillers in a game that didn't need filler material.

Also, if you're much older than 12 or 13 -- which I think was my age at the height of my Zak fandom --, you might find the plot and the characters a tad too juvenile.

The Bottom Line
Lucasfilm's adventure department evolved in leaps and bounds after this game, releasing a classic a year, so in retrospect their 1988 effort may not look much.
But as the first Lucasfilm adventure game that requires more than just a casual approach, Zak McKracken's spot is forever secured in the annals as an important release.