As characters I like them (ok, I adore Erandur, he's one of my favorite characters in the entire game), but as followers... Farkas' skills are completely mismatched and Erandur only knows low-level spells that don't match the fact that he levels up to 50. He has the advantage of being one of few mage followers who switches to melée weapons when needed, and he IS efficient on lower levels, but when it comes to BEST follower... No.
I don't look at stats when determining best. Plus Farkas kills things pretty easily despite his flawed specs, and Erandur handles his own as a support role. Good enough for me. Especially when I give them upgraded gear. They may not be the best combat wise, but they're good enough. I don't rely on my followers to kill things anyway.
My list is Serana, Frea, Aela, Farkas and Erandur. For mainly story reasons, but they all whoop ass. Especially Serana Frea and Aela.