I'm going to assume capped characters, but would welcome discussion of lower levels.
First off, I think the answer is "don't waste your money" if you're a square or semi - 101 ranks MIU is like having 75 ranks (for CS) or 51 ranks (for SMR stuff). I don't expect that would be effective.
Likewise, I imagine even pures should avoid the semi spells, because the MIU bonus is halved for them. For example, spike thorn is a great spell, but even at 202 ranks MIU you get 42 "bonus", which I guess would be similar to 42 ranger spells - doesn't seem promising against a level 100 creatures.
Furthermore, running the numbers, it looks like it's a little more practical to get a competitive cast from a CS spell than a SMR spell, though I'm not sure about how all the bonuses work/scale.
Of course, the answer could "don't waste your money, you already have disablers and high-damage spells", but that's not a very fun answer.
So I propose:
Wizard: Dark catalyst (half advantage from 425), maybe bind?
Sorcerer: Immolation (full advantage from 425)
Empath/Cleric: Dark catalyst still? Earthern fury?
Thoughts? Has anyone tested these out?