Well, my first playthrough she was the follower that was rather hard to kill. didnt need a replacement until later, but i guess its just that chick who fights for you, who looks good in armor, and while Aela is hot i can imagine she smells like a dog... while muiri is a cutie, i mean u have to (spoiler) kill her ex to be with her...
mjoll has that other guy as a pet, camilla is a hoe, all the chicks form the temple of dibella wear those not-so-sexy robes, ysolda has a fetish for cats, all the other housecarls it feels like youre settling, cuz ur simply buying the house... after helping a few people around town. theres the miner girl from shor's stone but she looks dirty... so really I just dont know. all the other chicks include dark elves who an be hot but ehhh, and old ladies and other female followers like Uthgard who can shove a fist up ur piehole.
Lydia is the chick who you dont care about too much until she grows on u. Watches u sleep, whines when she carries ur crap, but swings at the first Giant Spider or Ugly Falmer who threatens u.
...personally. I'd have a wife in every hold if u ask me.