Probably Vulcanus, it's the simplest, very easy to scale up and gives you tools to make the other two near planets easier.

Very minor spoilers about what helps and why:

::: spoiler Big mining drills make fulgora easier since you deplete scrap patches at half speed, which is good because medium islands have limited scrap on them.

Artillery makes gleba easier since you can keep pentapod nests from existing in your spore cloud. :::

Imo fulgora is more interesting and I actually did it first, but I do think Vulcanus -> Fulgora -> Gleba is probably the "optimal" order for regular playthroughs.