If we're talking taking a mod from the game and using it in real life, I'd probably go with Dove Balm because it just adds some wings that are capable of flight with no unnecessary complications and doesn't actually change anything about your existing body. If we were theoretically living in a world where transformations like these were normal, however, I'd probably go FrostFire.
If we're talking about adding stuff to the game, I would really love to see an in-depth Simii transformative item that functions like the Treatment and has the potential to turn you into a ton of different varieties of primates with their own unique designs and skills, such as orangutans, monkeys, gorillas, baboons, etc. This would probably be too complicated and niche to ever make it into the game realistically, but it would be really awesome if it did.