I think that the machine gun is the best weapon due to the fact that it's pretty early, strong, and if it's level 3 you can hover getting places early and/or easily (notably the arms barrier) and makes some bosses easier (e.g. the core)
if your going for the spur then having your polar star (no matter what level) kinda sucks. it feel like you have 4 slots (2 if you only go for a minimalist run) imo