The answer is a subjective one. Nearly all players say the feeling of Traveller they got from playing the game came from their first exposure with the edition they started gaming with.
I started playing Traveller when it was just the Classic 3 books. No universe setting. I was making my own worlds and subsectors. It felt sci-fi-ish, but in a Traveller way because of the hex codes obviously. I then felt the Traveller part more when the two Library Data books were published.
So whenever I need to feel that Traveller again, I read through the Library Data.
I started in CT. But Mega feels more Travelleresque to me. It's the system I measure all the rest against. (And TNE and Mongoose come up wanting almost every time. T4 is so different that it didn't feel like traveller.)