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Here's the basic run-down:

Phenderix: Has the most spells, at around 250 or something like that. Spells can only be bought by certain merchants added by the mod.

Apocalypse: Has a pretty good amount of spells, something like 150. The cool thing is that these spells are sold by actual spell-merchants, like Farengar, so they're completely integrated.

Midas Magick: Has maybe 150 spells as well. These spells are the most over-powered and also the most fun, such as Bug Swarm or Polymorph rabbit. You have to make these spells by using special "Aurum reactors".

I personally use the latter two spells mods, and they go great together. The first one is probably compatible too, because all the spell mods have different ways of getting the spells.