Good knives. I hated prep before that, now I can endure it (not really, I enjoy working on the cutting board as long as I'm not cooking for 50
Perhaps the most used items in my kitchen are the 2 restaurant quality half sheet pans that I use for everything from spill catchers under casseroles (before these pans it was either foil or run the cleaning cycle on the oven, often both) to cooking pans for free form breads to prep trays to cooking chicken parts. Once I even used them for baking a double batch of lemon bars, and that will be happening again next month, both times for local fund raiser benefit dinners.
Maybe my BIG stainless steel bowl. It's huge (15" across and almost 7" deep) and I can mix or stir just about anything in it without slopping over the sides. Along with that is my 14" nonstick frying pan... for the same reasons. I never have to worry about crowding stuff when I use that one.
Other things are definitely conveniences, but those first two changed how I actually do things more than most others. Most of my kitchen convenience stuff I just bought as I needed it, so although they opened new horizons, they didn't change the way I did the things I'd always done as much as those first two did.