Ask HN: What tech stack would you use for a brand new webapp?

I'm a pretty experienced developer (15 years in the industry at well-known tech companies), but lately I've been in management and while I still code in my spare time, I'm not totally up to speed on the newer stuff. I have some new side projects I want to work on, and I'm curious, in 2015, what tech stack would you build a brand new webapp in? I'm not asking what's "BEST" — I'm just asking what you personally would use if you were starting something new today, and why.

By "webapp" I'm thinking of stuff in similar size and complexity to something like Wunderlist — fairly simple, but with a nice UI and maybe eventually a companion mobile app. These will be small projects I do on my own for fun. No other developers to worry about, and I'm not looking for or expecting massive adoption (though I'd rather not use something that's unable to scale, in case I get surprised).

I've worked with a lot of languages over the years (including C, C++, Java, Python, Perl, PHP, Javascript, and a little bit of Go and Ruby), but I'm willing to learn just about any language and frameworks that make sense.