at HQ 5, tatooine is the best for rebels. I play 3 main accounts, HQ8, H10, and HQ10. For 2/3 bases, I stay on tat most of the time. Easiest to get higher in conflicts because that's where all the new players start. For the 3rd base, I typically stay on hoth because I unlocked arctic jetpacks only, not desert or forest jetpacks, but at HQ5, you don't have to think about the armory or jetpacks since that comes at HQ6.
If you move off of tat at HQ5, expect much better players on just about every other planet. The thing I don't like about the other planets is it is much harder to get top 20%/top 10%/top 2%, etc. in conflicts because seems as though every other planet except tat has a smaller quanity of active players. My $0.02, YMMV. Hope that helps.