[Refplace]

I dont really consider all those to be supermages.
I like all the systems but each has their drawbacks and advantages and I have made up my own as well.
Powers as magic is generally more balanced and better at high point totals.
RPM is great for versatility but can slow down play, especially if the mages player is not adept with the system.
Threshold Magic gives it a more strategic feel. First time I used it (back when it was UMana) the players went crazy and cast powerful spells like maxed out missile spells. Didn't take long for them to figure out they needed to manage their limited resources better.
Book/Path Magic is great for secret magic or magic is common but not key to adventuring campaigns. However spell design is a weakness.
Realm Magic is where we get into Supermages. However what we have is again a bit limited in new spell design.
Symbol Magic has flair but creating your own lexicon is time consuming and a creative hurdle. Also its again weak on spell design.
Basic Magic is great but the existing colleges dont suit my taste. Designing new spells is a bit arbitrary but there are a couple of pages of rules in GURPS Magic. This system has a few problems, most addressed in Wizardry Refined and the DFRPG but I loved the skills as spell system. However it is IMO too powerful as is at high point totals compared to a powers character.

I prefer kitchen sinks myself so tend to mix systems. In my current fantasy setting I mostly have Powers based systems but also a special system called Affinity Magic that is based off Ritual Magic (not RPM) with its own way of creating colleges. That makes it great for divine magic as I can have a war or agriculture college or narrow colleges like hawks based on a deities portfolio.
I added Book/Path to that to make for a peasant and common magic used mostly in ceremonies.

One thing I think you missed is mass magic, especially Cone of Power and Stadium Magic in GURPS Thaumatology. Also Ley lines from that book and Urban Magics.
I also tweaked ley lines to work with other sources as a power boost. Was in the Pyramid slushpile.