In the end, it comes down to:
Simple systems in game, with crunchy characters, with some background history, built in story arcs, and dramatic conflict.
I feel that 80-90% of a game is going to be determined by the referee's skill set, not by the rules, unless the rules as written are totally broken from go.
I own them all. I have played them all, except T20.
I like T4 a lot, despite everything everyone has said regarding it's problems.
But if it was 1977, and I had just seen Star Wars, and said to myself, knowing all the RPGs out there (today, via time warp). I'd pick CT.
Simple, doesn't tell you what to do as regards setting, and UPPs allow you to create anything. Ships are big enough for characters, and can do larger battles in a pinch.
The way subsector maps were done, you can in a glance see what is what and where and who has what power, trade routes, all sorts of stuff. 8 X 10, 40 worlds on average, was just a brilliant masterstroke by Marc. A whole year of gaming, in one small sheet, And it can be rolled out in a few nights, or added to as you go or expanded. Elegant is a word.
Combat is gritty and deadly, and you can reroll a dead pc in a few minutes, by looking at a single chart, not having to look up or make up huge stat blocks, or spend skill points.
As you roll, you sometimes select things to guide your PC career choices, but as in life, sometimes choices are made for you by luck, and you do the best you can.
A character sheet is one half of a page.
I used to have 2d6, a spiral graph paper notebook, a subsector map, and my softbound "The Traveller Book", and I was good to go for any game session. Draw maps, make it up as I go, whatever.
Today, a few hours ago, I ran a session of 3.5 D&D.
We needed to reference: PHB 3.5, DMG 3.5, MM3.5, Races of Destiny, for a question regarding half ogres, Races of Stone regarding dwarves, Arms & Equipment Guide for gear.
I just thought, pfft this stack of 3.5 books, I think I have 40 OGL books. I need crates to keep track of it. I fondly recall going on deployment with my little Traveller Book gaming kit. I could not do that nowadays.
I miss those days, and Traveller.
Analyzing games these days, it has again swung towards full bore complication, just like rolemaster did in the late 80's, and Nowadays, I have no time to read it all.
When I had my Traveller book, and 2d6, I had the keys to the freaking galaxy in my hand, and knew it.
I got lost along the way, lured by various systems. Anyway...