Eola is an odd follower. She has the lowest level cap of all the mage followers but she can cast the most powerful follower Destruction magic spells (Expert as oppposed to the Adept level spells to which Marcurio and the College of Winterhold mages followers are limited) and she also uses Conjuration spells to summon Flame Atronachs and reanimate corpses. Because of her low level she can burn through her magicka pool very quickly so you may want to give her lots of restore magicka potions. The other thing is she's a bit buggy and sometimes won't engage enemies.
Another Daedric Quest mage follower with similar abilities is Aranea Ileith, but you have to get the White Star which is not as useful as the Black Star that you give up as a result.
Illia is another good follower you can get at Darklight Tower at the end of her quest. She also can cast an Expert level Destruction spell (Ice Spear) and has a higher level cap and larger magicka pool. She is bugged however and the first time you dismiss her as a follower will also be the last time you do because she resets and you lose all favor faction with her making her unrecruitable afterwards.
For melee followers, the only outstanding thing about Mjoll is the fact that she can't die due to her essential NPC tag. In terms of pure combat ability though, the Housecarls and Vilkas are the best melee fighters as they all have higher level caps, better Block and Archery Skill and more Stamina. Although Mjoll is unkillable, they also have more Health which means they won't take a knee as easily as Mjoll. The Housecarls also have more health than Vilkas and can block more damage with their shields. If you give them Spellbreaker that will also make them highly resistant to magic attacks. In addition you can get a Housecarl very easily since you automatically get Lydia after you complete Dragon Rising. While Vilkas is a great fighter you wont be able to recruit him until you become the Harbinger of the Companions.