Isabela is definitely the funniest, but I'd have to say that my favourite DA2 companion is Aveline. She's a tough, straight-talking, no-nonsense kind of woman with a strong sense of right and wrong. But, she's also able to change her mind and be flexible: although she firmly believes in the law, she's willing to bend it if necessary to help others. She's very loyal to her friends and is also secretly quite the romantic, as is made evident during her adorable side-quest to woo Donnic.
Plus, she is the tank to end all tanks. The Immovable ability turns her into an absolute juggernaut.