With my first a boppy. He was a c section baby and it kept him off of my incision. Plus he didn't like being cuddled so it gave him a little space.
for my second it was aiden and andis bamboo blankets. Big enough to throw over her stroller when she was sleeping to keep the sun off, great for swaddling, great for diaper changes, you name it. They are soft and thin enough to fit anywhere. Shes 2 1/2 and I still use them.
For me it was some extra large swaddling blankets. But I've found that this varies so much between mothers (and babies). My son hated bouncy seats but loved his swing. I thought I'd wear my baby everywhere but soon found that didn't work for me. My sister gave me the travel system she loved but I found I'm partial to umbrella strollers. I barely used my pack-n-play but had countless people tell me it would be our best baby purchase.