I think it's also worth noting that Doctor Strange isn't simply "a powerful wizard" - he's usually dealing with issues that are in a completely different league from what most characters face on a routine basis. In-game, he might be over Level 20 - maybe with levels in the Archwizard prestige class (not to be confused with Archmage mythic tiers).
He really can't be that powerful a wizard, if he can be replaced in his job, by either Doctor Druid, or Brother Voodoo.
Doctor Strange stands out because he is a powerful wizard in a world where magic is extremely dangerous, rare, and practically unknown.
In a typical gaming world... he'd just be one of the working stiffs in the arcane line.
Ehrm... no. Strange is THE sorceror supreme of his dimension AND the champion of the Vishanti (arguably gods/archmages who reached godhood eons ago). There are many pratictioners of magic in the Marvel Universe. None is as skilled and powerful as Strange. The only opponents he needs to face carefully are the likes of Dormammu, Shuma-Gorath and Mephisto.
Also keep in mind Strange is immortal (as in he doesn't age anymore) and is more than 5 thousand years old having fought for that span of time in the War of the Seven Spheres, a conflict between the Vishanti and the Trinity of Ashes.To get an idea of how powerful classic Strange is consider he was able to stand his ground against Galactus and once even defeated him using the Images of Ikonn after the other heroes had weakened him enough.