This got me to thinking. My favortie art is Mentem. Sure, you get to bend minds and get everyone to do whatever it is you think, but that's just the icing. The cake is that you get to explore the most inner of inner sanctums, the inner world, and with that - with a cooperative storyfguide - you can tell tales of building memory palaces, traveling into the realm of dreams, and doing all kinds of spiritual stuff I just don't see working with other Forms.
My last character was a extraordinarily focused intellego specialist and he was a blast to run. The other characters (the players knew better)routinley referred to him as the most powerful and competant mage despite the fact that he really couldn't do anything outside of intellego (ok he could get off rise of the feathery body but that's about it). He just made extensive use of posing the silent question, eyes of the eagle, frosty breath of the spoken lie, sight of the tranparent motive and spontaneous spells to move his senses to diffent locations and was adept at unravelling plots in seconds. (this is the reason that although I had lots of fun I don't expect to play another intelego magus for quite some time).
The art that I keep comming back to time and time again is terram; build buildings and bridges, attack opponents with thrown rocks or created pits, walk through walls, conjure walls and ramps, etc. There's just so much to do with it.
Terram is just so fun and versatile that I have to go with it. Sure, you can throw huge chunks of rock, but you can also move most other things with it as well, due to its control of solid matter. But wait, there's more!
You can throw pots out of marble or iron with nary a seam to be found (using Rock of Viscid Clay and inventing a variation for metals). You can craft perfect armor, swords, jewelry, and the like. You can turn the floor of your lab into dirt to allow plants to grow naturally. You can create outlandish architecture or incredibly ornate jewelry. You can carve directly into stone or metal, making you the ultimate engraver (and without the messy dust). You can become clean with a simple perdo or rego terram spell, blowing the dirt from your clothes in an instant. You can Rego Terram and lift a rock or chunk of earth for use as transportation. I like versatile, and Terram is just that.
Muto has a multitude of uses, and terram as well. i have found that Muto0 is the most potent combat technique. Not as a direct damage dealer (you cannot cause permanent damage with it) but as a system to hinder the enemy: make the enemy a worthless fighter and allow your grogs (also increased in killer power through Muto) to deal with the mess.
It helps that you are not commiting a mortal sin either, since you are not killing them.
Muto also helps you increase your abilities and do all kinds of funky stuff to make your life easier.
Terram is my second choice for the reasons mentioned above
My favorite character is a Rego master and while he could probably control people he spends most of his time teleporting or flinging stuff like a jedi.
Another character I built is powerful in Mentem and Imagonem. I built him with the premise in mind that I wanted a mage that was hard to play. I wanted a real challenge and he has few spells that can cause physical damage. He has become quite a useful Magus that others turn to for a lot reasons- just not fighting.