First song choice is pretty subjective. It either depends on what someone wants to learn, or what the book/course they follow teaches them.
I'm currently reading an interesting book on how we acquire and improve (or not) skills. It's called "The Talent Code", and in spite of this overly catchy title, it's actually a very good read. Only halfway through, but it's already showing me that my approach to learning the uke (and everything else) is pretty sub-optimal. Apparently, the difficulty of what you learn is secondary, and "all" that matters is that you break it down to individual parts that you then individually learn (the way I understand it: you start with individual notes that your chords are composed of) at a painfully glacial pace, but with almost perfect accuracy/correctness, and then combine groups of individual parts into increasingly bigger groups ("chunking"), like building a house. I think the real difficulty with this approach is to actually take it this slow and to go really deep into the details.
But anyway, I'll write more about this when I'm done with the book and have formulated my plan how to apply this to ukulele practicing!
"Somewhere Over the Rainbow" is probably one of those songs that a lot of ukulele players learn (or try to learn!) early on. It's a song a lot of non-players associate with the instrument. Ukulele culture is largely about "do what you want and have fun doing it", though, so while that sometimes means lack of direction, it also means plenty of freedom and absence of pressure.
