Best gifts for each Hex member in Warframe?

Don't buy gifts blind. The item descriptions in the gift shop say who they're for, so there's no guessing involved. The general shape is that each member has a couple of favourites and everything else gives a small bump. Also the gifts aren't the main source of standing with them, the daily chats in KIM do more over time, so I'd put the gifts toward whichever member you're furthest behind on rather than splitting them evenly.