>>279827265 (OP)Fate is the one where he has the potential to shine the brightest. His life will be hard, but he has an eternal star guiding his path. Walking the same path as King Arthur, his life will be short but he may achieve something extraordinary before he dies.
UBW has him tempered by Rin's presence. While he doesn't have a star to guide his path, he has someone by his side through thick and thin. He may be able to go further than Archer did, without falling prey to the same pitfalls.
HF Shirou just settles down with Sakura. He probably won't achieve the same things as the other two, but he'll still lead a fulfilling life in his own way.
Incidentally, Nasu's stated a few times that he can't really imagine Shirou living a long life, so I'd imagine that no matter what path he chooses, he'll probably die before his 40s. Maybe HF's puppet body will fail at some point.