If I could only choose one, she'd be Fumino Furuhashi.
She's from Bokuben:
Bokutachi wa Benkyou ga Dekinai
But if I could have another one (don't get me wrong, choosing only one is tough) she'd be Kanade Suzutsuki as I find her personality quite unique and amusing.
She's from Mayo Chiki!:
I usually start and finish an anime with 12-ish episodes in a single day, so usually I'm only watching one at a time.
If I watch like 3 episodes and find it to be too boring, I then drop it. However, sometimes it's not too boring to be dropped, nor nice enough to keep me watching nonstop. In that case I might start another one to cool off a bit.
But I agree with you, I'd rather only watch one anime at a time. The thing is, sometimes there are situations like I mentioned where either I do this, or I drop the anime.