I voted for Star Frontiers, although it's worth noting that it's not necessarily a vote for the actual game mechanics. I just always loved the setting and the tone. There's serious nostalgia going on too.
As the vote is specifically for system, I did not vote for Star Frontiers which holds a special place in my heart, but for the D6 system which is the scifi system that I have got the most mileage out of.
I can hardly wait to find out what you're going to use. I've go Antarian Goose Bumps! They're yellow and hard as rocks like Antarian Geese!
I voted Star Wars D6 as my favorite, though X Plorers rocks mightily.
You should try Mongoose Traveller. It is a very sleek ruleset, very easy to use and very versatile, though it tends more towards the 'harder' spectrum, of sci-fi than to the 'space opera' side of things.