I had twins in 2015 and a singleton in 2017, so I really had a chance to look at what worked (and what was gimmick) with the twins, and what I loved with all three (or changed, once there were three).
I agree 100% on the Gerber onesies! Great for the price and make excellent layers under outfits or PJs.
My all time favorite items (workhorses), though are:
1. Wide-mouth, waterproof diaper backpack (lightweight and easy even with a front carrier with baby) the generic ones on Amazon are just perfect and come in many pretty colors. About $35.
2. Baby Bjorn bouncer – no noises, no motors, no batteries, and can be used from birth through toddlerhood. About $150, depending on colorway and sales.
3. Summer Infant Pop n Sit – about $25. This is a fabulous portable high chair and tiny kiddo beach/lounge chair! Worth every penny.
4. Stokke Tripp Trapp. I love that my kiddos could use independent and attractive high chairs, and then sit at the table once they could eat with dishes and utensils, and I love that these chairs will fit them into adulthood. About $250-350, depending on accessories purchased.