I've never been to any, seeing as I live in the middle of nowhere when it comes to hotspots. If I had to choose one, it would be Central Park, New York. Not just because it's a small patch of green in the heart of New York, but because it's the only one. Meaning it's the one stop for birds on their way north. Some even use it as a spot to breed!
South Florida promises vists from exotic birds, like the Budgerigar (of which I have two) or the Common Myna. This hotspot is the best, seeing as it has the most bird variety.
Cape may, New Jersey offers Golden-crowned Kinglets, Common Redpolls and Red Crossbills.
Southeast Arizona also has some cool ones: Groove-billed Anis and Spotted Owls.
I'm not saying that you have to go to any of these; I'm just suggesting a few good birding places. Even I haven't been to any of these! Just remember these places, and try to visit one or two if you can. Happy birding!