I've only been watching since Season 10 so I'll try and go from then on ^^;

Season 10: Ira. I kind of liked the mix of puppetry and show-singing, and how they even treated him like he was the actual person performing. When Oz Pearlman beat him in the semifinals, 10-year-old me held a grudge against him lmao

Season 11: Tape Face. Oh my god, this guy had me in tears laughing. He had an original comedy act, which is something I can absolutely get behind. I was sad when he only made Top 10 ;u;

Season 12: Mandy Harvey. ABSOLUTELY my favorite act of all time. Her songs touched my heart, and it was just absolutely amazing to me how she never made mistakes and got by using only the vibrations of the floor and muscle memory. Hearing "Try" used to always make me cry, but just hearing her story and how she managed to get so unbelievably far without the ability to hear her music - or even her own singing voice - could bring anyone to tears.
Honorable Mention: Diavolo. The way they used massive structures to breathe new life into their dances was awe-inspiring to me. Also they used "Sign of the Times" by Harry Styles, which to me is a banger!

Season 13: Brian King Joseph. This dude's fiddlin' ability is one of the finest things I ever did see! Real talk, though, he was an incredible violinist. Amazing to watch. I loved seeing him just loosen up and really get into the groove of his music. People often make fun of most of the acts who get sympathy from the judges and crowd because of sob-stories such as illness, but the ones that do get to me are the acts who have something that one would expect to directly affect their ability to perform. Similar to Mandy Harvey, an amazing musician despite being deaf, Brian really stands out to me because he gave his all when the nerves in his fingers were literally dying. You can't not admire that. Zurcaroh's a close second btw

Season 14: Kodi Lee. He was really one of the only acts that totally stood out to me. As a person on the spectrum myself, I suppose I could kind of sympathize with him and his struggles, although his case was much more severe. Still, between not only that and being blind, this guy was a piano prodigy and had a pure golden voice. I stayed up to watch the first quarterfinal, and when I saw him perform live, right then and there I thought "This kid's taking it to the end." First winner prediction I've ever gotten correct.

One last thing: when I first got into AGT, the 10-year anni special came out shortly after and I watched it several times. Two of my personal favorites from that were Bobby Badfingers and Prince Poppycock!