I disagree with @geigerm on this one, although there is no right or wrong answer.
The base game was great, but has some flaws, it favors the player that goes first with about a 5% edge, and Machine Cult (red) is overpowered.
The first full base expansion, Colony Wars, is great but it favors Star Empire (yellow) a bit too much and probably Blob (green). It’s a much faster aggro game overall.
Frontiers, is the most balanced of the 3 base sets.
Personally, my favorite way to play these days is all 3 base sets or Colony Wars + Frontiers
I would recommend getting the 3 base sets first, and then adding expansions in for flavor later.