Best baby/toddler products?

I was thinking today about how I wish I would have known about unnecessary products we spent our money on and products that would have really come in handy as parents but did not purchase. 
I thought it would be helpful for those of us that are already moms to list our number one must-have product we've bought, at what age you/your child used it, and list why you love it. Also list things you did not care for a why. Let's be helpful (not rude), ladies! 😀 I'll list mine below.