I usually take a mage as a follower, so my two favorites are Brelyna Maryon and Onmund.
Onmund is adorable (in a nerdy sort of way), doesn't whine when you ask him to carry things, and most importantly his lightning spells are amazing. He's been great with my Khajiit archer. I've never had an issue with him blowing our cover while sneaking like I have with other followers. I often marry him as well. As he's almost always with me it seems a pity not to be getting money, homecooked meals, and lovin' out of the deal.
Cons: he has trouble with fire, but a good Resist Fire enchantment will solve that. He can also be a little TOO protective at times - because he's so quick with the lightning hands, if you get into an altercation with a city guard you have to be very quick to put your weapon away if you want it to end peacefully. Otherwise Onmund will lose his s--- and kill every guard in town.
Apart from that, he's great.
Brelyna Maryon is also adorable. (Apparently my main criteria for a follower.) It's fun to listen to her, she's amazed by everything she sees
Whereas Onmund is mainly Destruction, she specializes in Conjuration, which can be useful if you're encountering enemies in numbers. She's also good about sneaking with you. As a Dunmer she has a natural resistance to fire.
The only real problem I've had with Brelyna - sometimes she can be a little slow to cast her conjuration spells when you're being attacked. She also seems reluctant to use any actual weapons you give her, even when its in her best interest. She will use a staff, though. I like to give her the Wabbajack and let her have at it.
Based solely on magical prowess,
Marcurio would actually be my follower of choice. If it weren't for
every word that comes out of his mouth. I find him unduly arrogant and childish, and if you ask him to carry something he throws a fit...I listen to adults having temper tantrums all day at work, so that's the deal-breaker for me.
