MtA nut here, so this comes as no surprise:
I've never had the chance to actually cast it, but it's just too awesome. Most of the times I depended on mind tricks - either the scrambling or the enhancing kind.
I LOVE MTA!
Though I think my favorite magic system by far has to be the one in Unknown Armies, everything has a cost and depending on your school of magic you enact a payment for your power. To sum it up it is guided by three laws.The Law of Symbolic Tension:
Your magic is based on a paradox.
((Example: if you are a Entropomancer (Chaos Mage) you master and control chaos by surrendering yourself to it completely.))The Law of Transaction:
You got out of it only what you put into it.
((Example: Billy Bob needs a bit of luck, but he is all out of charges, but luckily he is in a casino. He has fifty dollars left in his wallet and decides to bet it at a whim, not knowing the outcome. Unfortunately the number her called was not the right number on the roulette table, but luckily since he put himself in a minor unpredictable situation he generated a minor charge and has enough juice to pull off some magical mojo.))
Charges are like mana but instead of using a universal mana point they come in three levels.
Minor: You engaged in an act that matches your Paradox and obsession in the most basic and simplistic form and for putting in a little work you get a little mojo.
Significant Charge: You put in a lot of work into your Paradox and Obsession and you git a powerful chunk of mojo back.
Major Charge: You did something on the massive scale that matches your Paradox and Obsession and you get a massive chunk of Mojo Back. ((These are really hard to get in game and when you get one you can do some crazy stuff, typically a campaign objective is receiving a Major Charge.))
((For instance Billy Bob want's a bit more power then a Minor Charge, so instead he does the next best thing he pulls out a gun and begins robbing the Casino. Sure he is only one man and Casino's tend to have a lot of armed guards, but if he survives this Chaos will repay him in kind and he will get a significant charge.))Law of Obedience:
Your magic is the only magic for you.
((Example: Each magic is personal since magic is powered by an Obsession and a Paradox, Entropomancer's are obsessed with magic, Bibliomancers are obsessed with books and knowledge, and Cliomancers are obsessed with history. No one Adept is alike, there may be three Cliomancer's in a room but they all have different world views and their magic is heavily personalized.))
I hope I am not making it confusing it is actually a pretty simple system. >.<