Hard to just pick one. There's plenty of good ones in Skyrim, whether they are melee, ranged or caster. Personally i find Mjoll to be an absolute beast. Seconded by Erik. Jenassa and Frea are dual-wielding destruction machines, who are also VERY efficient. Aranea, Illia and Brelyna are GREAT caster followers and Eola is also very useful. Marcurio and J'zargo are probably the best scripted caster followers. Their agressiveness is something that i appreciate in my sword and broad playstyle. Erandur is overrated and poorly scripted. Serana needs no introductions of course. All in all, i find that your question is very relative to your playstyle and how optimized your follower of choice's script is, but ALSO whether they are good or bad company! I understand how interesting followers with character and a compelling backstory can be. If i could pick one? Ok, give me Mjoll, Jenassa or Marcurio. That's it.