So much hate for poor Lydia. I think I'm like one out of 1,000 players who actually like her. She's the only follower I use anymore because she can level to 50 (same as all the Housecarls). Hirelings/Mercenaries and other followers you get by doing favors for top out at levels 40 and 30 respectively. J'Zhargo is the only one that goes higher than level 50--he can go all the way to 81. But the problem with him is that he's largely ineffective on Master difficulty due to Destruction not scaling. So his spells aren't that effective; and when he runs out magicka all he has are his claws to fight with. So from that standpoint, the Housecarls (Lydia included) are the most effective followers in the game.
Aela, Farkas and Vilkas go to lvl 50 as well. At least Vilkas are pretty darn good when he's over lvl 40. Just saying.
Personally, I hate Lydia because she sits in my bedroom ALL THE TIME. I wanted my privacy back, so Lydia must go (and I pissed Delphine off so I couldn't send her to the Blades, either). I don't like her attitude, either. I mean, she's not the only follower to whine, but for example Marcurio whines in a fun way (you know, "I'm not a packmule"). Lydia just seems to hate her job, or me, or both.