There are four main versions, as far as I am aware:
1 - Theatrical release - this one has cheesy voiceover narration that explains every plot point. Unless you need things ELI5, avoid.
2 - Assembly cut - first go at a "director's cut" without the voiceover. A little rough.
3 - First Director's cut - on the success of the assembly cut, a proper director's cut was made.
4 - Final version - Further tightening up of the DC, including some new footage (but not in a Lucas'd up way). This is my recommended version.