Jericho: Best attitude, best at flanking. There's something comforting about knowing a bigger psycho than you has your back. He's my favorite because he doesn't judge. The bigger the mess, the more he cheers. He only grumbles about lack of action or humping it across the wasteland with loot. He lives to destroy. It's psychically liberating to not have to behave. I also found him incredibly accurate with missile launcher, Minigun and with the Chinese Assault Rifle. The latter uses his default ammo, not my precious loot. You really feel like you're working with a teammate; plus, because he dies relatively easily, you need to work together.
Charon: Tough and limited, but predictable. I like his grumbling, as well as his horror at an evil character's excesses. Takes a lot of damage, looks great in his Enclave Hellfire Armor and becomes even harder to kill. Sometimes he seems to get lost on me, which I find to be a drawback. And, because sometimes I mistake him for Enclave or, if he's unmasked, a ghoul -- and I shoot them on site.
Sergeant RL-3: Great agility, love the plasma cannon. But you can't cross the yellow line.
Fawkes: Incredible power, almost unkillable, best weapon of any follower. But you have to behave.
It depends whether you have a criteria for "best."
^I've been using her and so far she kicks ass. Give her a really good laser rifle and she'll pick enemies off left and right. Plus I've noticed that when enemies hide and she doesn't have a shot, she throws grenades at them instead.
Keep in mind that I haven't used Fawkes yet b/c I can't find him......or her.
Why doesn't Dogmeat count? He's my favorite with Broken Steel installed. I just remain in stealth and let him distract them all while I take cheapshots at the enemies. And the non-inventory doesn't matter to me.