SCM is a very VERY close second imo. In terms of versatility it probably ranks higher (depending on the time scale, as a dweomerkeeper, given enough time, has access to more spells/abilities than even a SCM, that is to say, dwes have access to ALL of them), but, and correct me if I'm wrong, dwes are the only class that can pump out free I've assassins without the assistance of items.
All those twinned fusions and celerities are nice and fancy (although without favor of the martyr is arguable that only one celerity would be usable before the daze kicks in) but they are utterly useless if you don't win initiative. All you need to win any Mage battle is a successful time stop, whoever gets that off first wins.
Either way I'd rather play my SCM that uses circle magic to get her CL up to 50 something and uses her miracles to make simulacrums of pretty much anything. Moreover, due to clever usages of echoing spell, miser with magic, rapid metamagic, and meta magic reduction she effectively has unlimited spell slots.
Free ice assassins = access to any spell or ability in the game.
Supernatural spell at 21 = any epic spell you can think of creating for a measly 9k gold. This IS pun/pun mark 2. Between the infinite stats, immunity to all spells, access to every SU ability (due to ability rip; non-epic, but epic can make them last indefinitely), the only thing this doesn't achieve is the arguably TO ability of pun pun's to make up abilities (which I think is total bs and only one insane reading of manipulate form). To date there has not been another method of achieving this level of power (yeah there's a few infinite stuff out there but nothing that gets everything in one character like pun pun)...so unless I'm missing something tainted scholar/spell dancer/illithid savant have nothing on this (I suppose illithid savants can gain supernatural spell from a dweomerkeeper but it still requires the class to exist and it's still the dweomerkeeper's ability...either way, it's like 30th level by the time you can do what you can do with a standard dwe at 21st; not worth the delay).