You know, I am a big fan of the Fate/Stay universe and I couldn't decide betwen Ilyasviel von Einzbern or Rin Tohsaka (There are other ways of spelling it, but essentially the sound is what matters)...
Illyasviel von Einzbern summons Berserker (Hercules) and essentially dominates the Holy Grail War but never wins in any of the arcs, although she is simply the biggest boss until later on in the stories aside from the Heaven's Feel arc.
I don't want to say too much about her in case I spoil it for others, which is sucketh. I'll be honest, I hate it when others do it to me.
Rin Tohsaka summons an Archer class servent whose hero is from the same timeline when the story takes place (as vague as I can make it without giving it away), in the one arc she manages to create a (I think the term is 'final magic') Final magic of the gem magicks, the Gem Blade. She wins out in the Unlimited Blade Works Arc, and does fairly well in the Heaven's Feel arc, but the main arc (Fate Arc) between the protanganist (Emiya Shirou) and the Saber class (Arturia Pendragon) she is essentially the one who tutors Shirou along at points.
I would've said Shirou but to be honest, his shining moments are usually at the end of all the arcs, each time he did a sepearate, yet damn amazing things...
Okay maybe I will mention the guy.
Hmmm, are there spoiler tags?
Well, since the Fate and UBW arcs are already out and about, I'll mention those but Heaven's Feel you'll just have to read the Visual Novel for that one.
Fate arc: Projection of the Legendary Blade, Excalibur (if I remember correctly)
UBW: Manages to successfully start up the Reality Marble (a reality of the mage projected upon the actually reality of the world) called "Unlimited Blade Works" and has an awesome battle sequence. Good line as well.
(Note: If the image is too big, I'll edit it out and just put the link there instead.)
I know it says one, but damn, Type-moon makes some decent mages in their stories. These are the ones that stick out (for me), but there are many others in their works or authors related to them.