I was around for a good chunk of these song releases, so I'll also include notes on what I remember from those years, who entered the community, what was going on, etc. I was considering including the Hitoshizuku x Yama and Machigerita songs where Kaito sings, but.even if they are popular, Kaito doesn't really have a lead role in them. Kaito's success was heavily influenced by Miku's release, and of course a lot of popular Kaito songs are duets with Meiko. There are duets/group songs and not just solos here, too. These picks are personal and based on my own opinions of what's "iconic". I chose songs that had a substantial view count (depending on the release year). I'm also not saying these are "must listens" or anything, just the most notable Kaito songs from certain years.
*Note: This thread is not made to shame people who don't go out of their way to look for niche Vocaloid content. There are definitely some songs I missed (I used VocaDB to find all of these), but I can always add more. Kaito is one of the oldest Vocaloids, so it was fun to track his growth over such a long period of time. But after meeting so many people who can't name a song with him that wasn't from over a decade ago, I thought this collection might help a little. I made this list for an anniversary project I'm doing, with no intention to post it. Yeah, Kaito is one of the most recognizable Vocaloids, but in 2020 it's rare that I (personally) find people who actively seek out songs with him.
I've been a Kaito fan for 9 years now, and in that time I've seen almost every opinion on him there is.