I have no experience with mage classes first hand but from what I can tell from partying with them, I'd say they would all fair about the same in soloing. Summoners just don't seem to do very much damage to solo very efficiently speed wise.
I'm going to assume the most strenuous choice which is that you want to full solo King's road on stages at your own level. Depending on what you're looking for in your runs, be it efficiency without the aid of pots or just speed, I would suggest different things.
My grappler I find to be the fastest farming class in the game. The mass amount of AoE and high damage is just riddled throughout the class and at the same time you are very rarely open to attack. However, they also burn MP pretty fast so having a lot of MP regen equipment would be required.
AradSociety - Skill Planner
This build would be a good soloing I feel. Constantly having irresistible grab on and a high level intense takedown will really enhance your damage as well as making your AoE much larger. Some may question the high level suplex but with the level cap as it is, I feel it's the better choice. It's low cooldown and large AoE with intense takedown make it a very useful skill to have. Sliding grab is only necessary if you want to herd enemies using Spiral which I love doing.
Now if you want survivability in your runs and the ability to be the most self-sufficient as you can be, I would suggest Crusader. My crusader has only used 10 potions I think since I made him as I am built to constantly keep my MP and HP high. The only problem with soloing with Crusader is that unless you go battle (Which I feel for staging would actually make you go slower in terms of damage), your damage is entirely based on your weapon. Priests have a great basic attack so if you got a high plused weapon to use, Crusader would be able to plow through stages easily enough.