Penguin for lectures, books and USPCC cards. They do have a good selection with good prices and easy returns.
I've been leaning more toward Vanishing Inc lately as they are a little more focused. Mostly books and card videos.
I already have a whole bunch of T11 stuff, mostly card videos and book tests. I'll probably be getting Medallions to supplement my premium card needs because:
I was getting Steamboats from Art of Play, but those seem to be discontinued. I am forced to live off of the six bricks I have horded away.
Conjuring Arts for my Superior Playing Cards fix. Nice cards! Check them out.
Pattrick's Magical Services for close up mats. Nice mats, prices and service.
I've also had good experiences with The Magic Apple, Steven's Magic Emporium, Browser's Den of Magic and Mad Hatter Magic. It's nice to support the brick and mortar shops, but they cannot keep as large an inventory. Though they will often have stuff the big guys don't.
I have had good experiences with them all, it comes down to availability and price.