I don't use flawed traits very often, but when I do, I lean towards Clumsy and Erratic the most. Oh and sometimes Neat. I know most wouldn't pick that as a 'flawed' trait, but in my eyes, it really is flawed. It makes for some very funny moments when those Sims are always trying to clean desks. The faces they make look almost demented in their quest to clean. It's doubly fun when another Sim is trying to use a computer on the desk they're cleaning. hehe
Besides those three, I have one more. In the past I avoided Glutton like the plague but tried it once on my collector Sim. It was so fun! I love the hilarious faces and sounds they make while enjoying their food. That Sim ended up being one of my absolute favorites, she had such a fun personality and I gave her the bouncy walk too.
Another perk with that trait is how they can rummage for food in trash cans. Yeah, it's a bit gross, but it sure comes in handy when you play Rags to Riches style like I do with 95% of my Sims. So, I'm going to say that trait is my new flawed favorite. :)