i'm very biased with my choices but
Blue lions > Black eagles > Golden deer/Church > Church/Golden deer
i feel like going with Blue lions first heavily impacts the first part of the game since each student is involved in someway, unlike BE and GD, and also it reveals the least about the lore of Fodlan so it can be explored in other routes.
Black eagles as the second choice works because you can see why edelgard did what she did and get some glimpses of TWSITD and what they're up to.
Golden deer as last because it reveals a lot of lore that frankly really does feel like something you should experience last.