I'm with bettaowner on this. The easiest is probably the dwarf/pea/malbar puffer, which is freshwater. The only reason, IMO
that is it easier is the freshwater aspect. As bettaowner said that was probably the green spotted puffer, super cute, but eventually brackish and need I think 30+ gallons... I've never had anything other than a dwarf puffer so my knowledge isn't that advanced when it comes to the others. Also, contrary to what bettaowner said, the dwarf puffers don't need
snails. For some reason mine actually refuses to eat them... Yeah weird.
If you want any help on the dwarf puffers feel free to ask! But if you're still looking into the green spotted puffers, I'm probably no help! Good Luck!!