I definitely agree with MusicallyInspired that you should play all of them. They make a very good, remarkably consistent series. But since you asked, my own favourite Space Quest games are SQ5 and SQ3, in that order.

 

Also, should you feel the need for more Space Quest after you've completed the official games, there are a few fangames out there you may want to check out. I wrote an article about the most noteworthy ones back in 2012 that should serve as a little introductory guide: http://www.spacequest.net/index.php/page/articles.html/_/ramblings/who-says-space-quest-is-dead-r11

Oh, and yes, by all means - do follow PurpleTentacle's advice and preorder SpaceVenture. I personally have little doubt it's going to be every bit as good as the Space Quest games and well worth the wait.

It's hard for Space Quest-obsessed people like us to pin down what makes each Space Quest game better than the other. So I've tried stepping out of my subjective navel-gazing and offer this handy guide:

 

SQ1: Play if you like being marooned on desert planets.

SQ2: Play if you like being marooned on jungle planets.

SQ3: Play if you like hopping around planets and playing arcade sequences.

SQ4: Play if you like hopping through time and being in constant mortal danger.

SQ5: Play if you like hopping around planets while solving a mystery of conspiracy and bossing people around.

SQ6: Play if you like hopping around planets, then some chick's inner organs.

My favorite parts are probably both in SQ4 -- the beginning, including the whole opening sequence and realizing you're in a future SQ,

and then when you get sent back to SQ1 was a laugh-out-loud moment as well as a clever idea and intriguing to

see what it would be like looking around SQ1 with the Roger from SQ4.

Nice sum up, Troels. I agree with those points.