So, it's an interesting, and one of the most common debates. I feel like it shouldn't be a debate, but whatever.

This isn't my take, by the way. It's a fact.

Avalon Artoria under Shirou vs Gilgamesh. Artoria defeats Gilgamesh 8/10 times (Happens in the Fate Route).

Artoria under Shirou vs Gilgamesh. Gilgamesh wins 10/10 times (G.O.B. spam till win).

Artoria under Rin vs Gilgamesh. Gilgamesh wins 6/10 times (Artoria has better Parameters, except for the Noble Phantasm and Ea, thus better win-cons).

Avalon Artoria under Rin vs Gilgamesh. Artoria wins 10/10 times (Better Parameters, and Avalon hax are crazy overpowered).