I like both when done well. Skeuomorphism can be dripping in charisma and personality and whimsy and warmth, and really add a lot of fun to your experience, but let it get in the way of practical considerations or get carried away adding details that aren't reflected in functionality and it quickly becomes annoying and/or slow to use.
My favourite skeuomorphism is in early 90s computer games — where it makes loads of sense because they went all in and had the textures and grains through absolutely everything on screen, rather than just the window UI — and the Classic era Mac OS — where it was more of a highly-functional and semi-flat lite-skeuomorphic vibe.
Flat is fantastic when it makes good use of colour/tone/shading, layout, iconography, and consistency, and it makes a lot of sense on our shiny flat screens — especially in a time when so many interfaces are designed for touch. The trouble is we don't often see all of those elements done well. Or sometimes we see it done well in a functional sense but completely devoid of personality and beauty (and flat can absolutely become beautiful and even whimsical in the hands of a skilled designer).
And as someone who's always used both Macs and PCs, I can get nostalgic about both approaches. (I had an absolute blast creating the DOS UI-inspired website for my Shareware Heroes book, for instance.)
So most of all I'd just like to see more good UI design, be it flat, skeuomorphic, or a hybrid of the two. But as for what the next big UI design trend might be, I'd say it depends on the extent to which spatial computing catches on — if that gets big then the dominant UI style will match whatever feels best to early adopters of that tech.