Sacrosanct
Legend
from any system. I'd love to hear it.
Naturally, as a game designer my favorite system is one I came up with
But I borrowed bits and pieces from other systems that I liked.
For a basic caster in a basic system, I prefer at-will magic. No Vanician magic or spell points. If you're experienced enough to cast it, you can do it. This often leads to a lower magic setting unless you have some sort of limitation put in place (like fatigue for the more powerful spells).
For a more robust system, I prefer a sort of hybrid spell point system that pulls a little from Earthdawn, where you have varying levels of success based on how well you made a contested dice pool check. This is a system I use for the "sorcerer" type classes.
How it works
You have a pool of stamina that you fuel your spells with. You can boost your spells by investing more stamina. When making attack rolls or a creature is trying to resist a spell of yours, you both roll your appropriate dice pools and compare the highest dice. For every one of your dice beyond the first that is higher than their highest dice, you get an additional effect (more damage, longer duration, etc.)
My other favorite system is a rune system, where each rune has a magical effect and you decide which runes to combine together to get a final spell effect. This offers a ton of flexibility to creative players.
How it works
Let's say you combine a burst rune, fire rune, and gust rune together. The result is a glowing flame rune that shoots out to range and then explodes. Fireball! Or you combine a healing rune, dividing rune, and gust rune to send out a rune that splits in two, hitting two targets and healing them both. The more experienced you get, the more runes you can combine into one spell, and with greater affect. One actual play interesting example was combining a portal rune, gust, and explode to create a series of portals in a large area, so each PC could enter the portal in one turn rather than spend two rounds of movement to reach it.
Naturally, as a game designer my favorite system is one I came up with

For a basic caster in a basic system, I prefer at-will magic. No Vanician magic or spell points. If you're experienced enough to cast it, you can do it. This often leads to a lower magic setting unless you have some sort of limitation put in place (like fatigue for the more powerful spells).
For a more robust system, I prefer a sort of hybrid spell point system that pulls a little from Earthdawn, where you have varying levels of success based on how well you made a contested dice pool check. This is a system I use for the "sorcerer" type classes.
How it works
You have a pool of stamina that you fuel your spells with. You can boost your spells by investing more stamina. When making attack rolls or a creature is trying to resist a spell of yours, you both roll your appropriate dice pools and compare the highest dice. For every one of your dice beyond the first that is higher than their highest dice, you get an additional effect (more damage, longer duration, etc.)
My other favorite system is a rune system, where each rune has a magical effect and you decide which runes to combine together to get a final spell effect. This offers a ton of flexibility to creative players.
How it works
Let's say you combine a burst rune, fire rune, and gust rune together. The result is a glowing flame rune that shoots out to range and then explodes. Fireball! Or you combine a healing rune, dividing rune, and gust rune to send out a rune that splits in two, hitting two targets and healing them both. The more experienced you get, the more runes you can combine into one spell, and with greater affect. One actual play interesting example was combining a portal rune, gust, and explode to create a series of portals in a large area, so each PC could enter the portal in one turn rather than spend two rounds of movement to reach it.