Clamp lights -- answer to both questions. They got me going, but now that I've used better lights (barndoors, focusable), I wish I would have saved up all the money I spent on my dozens of worklights/clamplights and bought a couple of decent 500w+ fixtures.
Worst? Probably all the sound gear I purchased. When I'm doing a production I have to get a sound guy in to help, because I'm not a sound guy myself, and most of them already have their own and better equipment that what I own so in the end it looks like £500 I could've spent elsewhere.
Best? DSLR Slate for iPad. Cost me about £7 and I've used it endlessly on various shoots. Actually having an iPad is a real blessing for a filmmaker because it's a really useful thing to have in pre-production and a much more convenient thing to have with you on set than a laptop or a proper computer.
I'm still relatively new, so mine will sound like a "so what" for best:
But when picked up external microphone-a Shure 608 (I had used Camera Mic up to that point) I was reminded how much clearer and more control I had of sound! It was wonderful!
Don't have a "worst"-other than I picked up a ceramic type lamp at a discount store for lighting, and first day I had it outside, it got knocked over and busted-so if that's "worst" so be it