KillerofGods -- Which order you crack the homeworlds has benefits and drawbacks. In general:
1. Easiest First: Pros -- You'll spend less time refleeting between homeworld assaults if you go for the easy one first, as you should take less casualties. Depending on whether you like to incorporate warheads into your tactics, you'll also use fewer of them, driving the AIP up less, meaning the harder HW doesn't get to juice up as much. If you play with auto-AIP on, the faster attack/recovery will also help to put you in position to attack the harder one at a lower AIP. If you don't nuke the HW, you'll be able to capture it and strengthen for the final HW.
Easiest First: Cons -- If you are already in a precarious position and barely hanging on, you might not be able to survive ramping your fleet strength all the way back up to full, and you may not have the strength to take the harder HW with a weakened fleet. If you play with CSGs on, sometimes you have no choice but to alert the core world(s) by the harder HW first, if it happens to have a CSG-A in a bad place, meaning you have to leave one or more very powerful planets on alert the entire time you take out the easy HW.
2. Hardest First: Pros -- If you survive this, you know you have a lower hurdle to clear on the last HW, which is good for morale, and it also means you may be able to skip fully rebuilding your fleet if you simply don't need full strength to destroy the last one, saving you time and possible AIP/exposure to more waves/CPAs/exos.
Hardest First: Cons -- You will take more casualties and burn more time on the harder world in the attack and in the subsequent recovery, meaning potentially more of a gain on AIP, more time exposed to counterattack, and greater risk of losing irreplaceables and not having their benefits available for striking the easier HW. If you use warheads, you'll probably burn more of them here and shoot the AIP higher than you would on the easy world. This is especially true if you choose to nuke the hard world and take the +50AIP from a Mk.I.
The next to last point to consider in general, is whether you can gain any irreplaceables from one HW or a nearby world that you could also use for staging. Spire Archives, ZPGs, fabs, etc. If one world offers one or more of these and the other one doesn't, it can be a good idea to go for it first since it'll strengthen you for both attacks, rather than only the last.
Last point -- you should also consider whether you have any "captured" ships enhancing your fleet, like stuff from Zenith Reserves, Botnet zombies, or flocks of reclaimed stuff swiped from the AI. Any of these will be difficult or impossible to replace, so consider carefully which world you want to spend them on.
