The endings vary in exactly what information they reveal, what actually happens (even down to certain events you wouldn't expect to change), and their overall emotional impact. There are five endings (not counting the ability to get no ending), and they are as follows:

• Kotori's route
  
• Amane's route
  
• Ageha's route
  
• Asa's route
  
• Yoru's route

The guide below orders these from first to last, and provides my reasoning why, as well as a quick guide on how to reach them with as few spoilers as possible.

When you begin, most options are grayed out. At this stage, it's near impossible not to pick one of two particular endings, with Kotori's being one of them. I recommend this over the alternative because it (to me) is one of the two most powerful endings, and provides a really solid introduction into the VN without being free with spoilers for other routes.

To achieve this route, just keep picking options that either compliment or allow you to spend time with Kotori. To save time later having to skip through the early parts of the game, make a save before selecting the first choice of the game.

I'm placing this second mostly because this route doesn't quite fit elsewhere. It's got a very different feel to all the other routes, and, whilst still excellent, I found it quite a confusing and irritating route*, so is best not placed in a memorable spot. Finally, it is the second easily attainable route at the start, implying that it is intended to be seen early.

To get this route, reload the Kotori route save, and select the Ageha option. Continue as before, skipping if you wish. The next choice reached is a new one unlocked by completing Kotori's route. Save here, and then nip any romance here in the bud (we'll refer to this save as save a). For the option after this, save again (and refer to this save as save b) and pick Ageha. For the remaining options, keep selecting Ageha.

*Only true if playing the unrestored version. They cut this route down so much it no longer makes sense.

The final option unlocked from the start, but very hard to reach without completing the game at least once. It's a very heartwarming, relaxing and cute route, so it works well after the Ageha route. Finally, this route provides some important backstory on a couple of characters who, up until now, have been fairly minor players.

To get this route, load up save b, and pick nobody. Maintain this neutral stance until options to choose or spend time with Asa appear. On the second of these, save before making the choice (refer to this save as save c).

Although the game refers to this route as such, it's not strictly true. You'll see. Suffice to say it's quite an endearing route that deepens the plots of the twins whilst providing the single most heartwarming scene up until now. It's worthy of being the penultimate route to be played.

To get this route, load up save c and pick the option that isn't Asa.

Save the best 'til last is always a good adge, and one I've tried to follow here. Amane is unlocked after completing Kotori's route, and is the other most emotionally impactful route in the VN. It also reveals the answers to a lot of the mysteries in the VN and resolves them wonderfully, which is the other reason I recommend leaving it for the end.

To get this route, load up save a, assert your interest, and then simply choose options that compliment or allow you to spend time with Amane.

I wish.

Thanks for reading my guide, hope it helps improve your game experience!
I worked this out through playing the routes in a pretty bad order that somewhat hampered my enjoyment, so I hope the same doesn't happen to you.

Happy flying!