First, a KitchenAid stand mixer. Despite the years I've been baking, I didn't buy one for myself until more than 12 years ago. I made do with a handheld mixer but finally broke down and bought myself a 5-quart Kitchen Aid at Costco when I found one at a good price. I LOVE my KitchenAid mixer. It made all the mixing so much easier and faster. I still remember when I first bought it, I baked one cake using the handheld mixer and one cake using the KitchenAid. The one using the KitchenAid turned out lighter and better - the difference was really noticeable. I'll never go without a KitchenAid again. They also last a long, long time. I put mine through frequent use since I bake anywhere from 1-5 times weekly nearly every week and have for years and my white one works just as well as when I first bought it. However, I did get a hankering for a pink one and started wanting one some years ago. But the practical side of me couldn't bring myself to buy a new one because my white one worked perfectly fine. Last Christmas, my old college roommate, Caroline, who's also another baker, surprised me with the pink KitchenAid I'd been wanting. I was floored and touched by her incredible generosity. I gave my white KitchenAid to my mom and have been using the pink one ever since.
My next favorite gadget is the nut grinder I mentioned earlier. No more laborious chopping. Just toast the nuts, let them cool and grind them. Even the good ones are cheap (less than $10) and they last a long time. Whole nuts are cheaper to buy so a nut grinder saves you money. I buy whole pecans from Costco, whole cashews and macadamia nuts from Trader Joe's, and my friend Linda gives me whole almonds - with the nut grinder, it's easy to chop up the nuts if that's what you need.
The third baking tool I find indispensable is a microplane zester. I used to make do with a normal zester but once I tried a microplane zester, I've never gone back. It zests quickly and easily and lets you get the actual peel without the white pith. (When you're zesting lemons or oranges, you just want to get the outer peel, not the white part.) They're a little tricky to clean since they can shred your sponge but just soap them carefully and rinse in warm water. I also dry mine as thoroughly as possible and if the oven is still a little warm from whatever I've baked, I pop it in there to aid in the drying.
Of course there are the usual measuring cups and measuring spoons that any good baker needs. As long as they're accurate and hold up well, you can get whichever kind you prefer. I have both plastic and metal sets. I find it handy to have a couple of different sets since I bake so much and use one measuring cup per ingredient. Having multiples saves time so I don't have to wash everything while I'm in the midst of measuring out all the ingredients.
ETA: I can't believe I forgot the most essential baking tool - high heat spatulas! I have several and use them for everything, mixing, scraping, stirring, etc. Very handy. Invest in a few good ones of varying sizes. I like the narrow ones and the regular size ones. High heat spatulas are essential, not the regular spatula kind. They're more versatile and can use them at high temps (hence "high heat" but you probably already knew that).