I have devoted an entire cabinet in my kitchen to baking pans. And then some.
It's a little embarrassing. Square pans in multiple sizes. Round pans in multiple sizes. The new loaf pans. The old, maybe-I-can-still-use-them loaf pans. I could go on.
I have surely gone overboard, but my hoarding notwithstanding, having a few different baking pans and dishes is smart and it's a worthwhile investment. Plus, these pans are versatile, so you can do much more with them than baking.
Here are my essentials and a few ideas about what you can do with them.
Rectangular baking dish. The most common size is 9 by 13 inches. You can choose from glass, metal or even ceramic, and what you choose really depends on what you think you'll be making most. Glass is better for all-purpose cooking/roasting, although you can get away with baking if you keep a close eye on your baked goods and/or turn down the temperature (glass is a better insulator than metal, so cakes can brown and cook through faster). Cooking savory dishes such as lasagna, mac and cheese, strata and casseroles in Pyrex (or ceramic) is better, because you can slice and serve straight from the dish without worrying about scratching the surface. You'll also get nice browned bits when you roast chicken with vegetables. If you're more interested in baking at that size – mega brownies, for instance - stick with metal.
Square cake pan. Typically 8- or 9-inch squares, although I think the larger size is more versatile (and yet I own both, oh, and a 7-inch too). Again, look for metal pans with right-angled corners (rounded make it harder to remove food from the pan, especially if you don't line it). This is the classic brownie pan, meaning it's also good for other desserts (cakes or bars). I also use it to bake one of my favorite coffee cakes. Square pans are a nice vessel for baked eggs, or a small casserole or batch of mac and cheese.
A loaf pan. The standard sizes are 9 by 5 inches or the slightly smaller 81/2 by 41/2 inches, and they're pretty much interchangeable as long as you tweak the cook time if you're swapping one in for the other. I generally prefer metal over glass for the same reasons mentioned before. I use my loaf pans the most for quick breads, which can lean sweet or savory. A loaf pan makes for a handy mold for a semifreddo, too. Use it for meatloaf, sandwich bread or terrines.
Round cake pan. Also available in 8- or 9-inch sizes. Pick whichever size you prefer (go for metal, though) and adapt recipes accordingly. If you think you'll ever be making a layer cake, consider buying at least two in one size. You can also bake breakfast rolls, yeasted rolls and focaccia in them. Or a savory cheesecake.
Pie plate. Standard size is 9 to 91/2 inches. You can buy cute ceramic ones, but I prefer good ol' Pyrex – it's cheap, bakes evenly and lets you see exactly what color your pie is. Yes, definitely make pies in your pie dish! Or pie-adjacent dishes, including quiche. I've also used mine to make shortbread. When you need something shallow for dipping (bagels in toppings) or breading (chicken cutlets, above), a pie plate is very handy. It's great for a gratin, too.