Probably Tuesday. Monday I have no energy, Wednesday I'm back to not having energy, Thursday is filled with me thinking about Friday, and Friday is filled with me staring at clocks. Tuesday really is my best day energy and enthusiasm-wise.
Edit; oh, we're talking about the GB crew? Fuck that! I'm going to keep talking about myself!
Sunday? How is that in any way professional? Sunday is the day that I prop my feet up, yawn, and play video games all day (and occasionally record a podcast). I can't imagine doing anything else on Sunday. EDIT: Except the paper due Monday that I've been procrastinating on for two weeks. Those never fail to rear their ugly heads.
Tuesday, though, now that's a professional day. You've gotten past the beginning of the week woes on Monday, you're not yet stuck in the despair of Wednesday, and you're not thinking about the end of the week like you are during Thursday and Friday.
So, yeah. Tuesday is like that mystical time between 10 and 12 o' clock, when you get virtually all of your work done.