A lot of people say its Dine because of the combination of high enemy spawns and a lot of loot with low value (on average, you get about 225 scrap worth around 12 credits). But Dine is also one of the most important moons for hitting extreme quotas with average profit value of around 2400 and a maximum around 24000 thats more than any other moon.
Ive also heard that Offense is the worst, but among the free moons, it actually has the best scrap amount and profit value.
In my opinion, the worst moon in the game is Embryon, mainly because of the Old Birds and the high risk combined with low scrap amount and low profit value.
Also I have two friends one of them thinks Adamance is the worst moon because of the number of Baboon Hawks and the maps terrain, while the other thinks March is the worst because of the maps extremely large map size for free moon.(Both of my friends are new and havent played any of the paid moons except Rend.)