Well I feel like Len has no personnality imo.MEIKO is easily my favorite cryptonloid and overall vocaloid.
I don't really have much reason for her being my favorite other than when I first got into vocaloid I wanted to find a more mature, natural sounding vocal, and when I started listening to her more I just fell in love with her tone and overall character with all the fanart and what not.
I also feel I need to mention that Miku is my second favorite just bc I love her so much<3 When i first got into vocaloid i didnt really like her that much, but my love for her has been growing over the years, especially since the fandom got a bit more quiet and i started listening to vocaloid more casually, i find myself listening to her so much more than anyone else.
He's like the kid in high school that is only popular because he does crazy shit/jokes around a lot.
You can't really love Len like the others, you like him and you can't go any deeper in the relationship.
but producers are still publishing songs for him tho (well... Neru is lol)