Shiro - Was never meant to be a magus from the beginning. He received no training, nor a magic circuit. The fact that he can do what he does is impressive on its own.
Rin - Fully-fledged magus, was able to stand up against a Servant-class magus from Ancient Greece. Didn't fail her summoning in the slightest.
Shinji - No disagreements here. Talentless to a fault. However, that's his family's fault. It's an all-round agreement that if he'd actually had the advantages of an actual Magic Circuit and the ability to cast magic, he could possibly have dominated the War.
Ilya - A homunculus of an incredibly powerful magus, and able to keep a Berserker'd Herculus in check using almost all of her energy, while still having some to spare.
The main problem with how the War turned out is a) They're all a bunch of fucking kids, and b) Kotomine was pulling a lot of strings behind the scenes.