It depends on what you use to define hottest. Most paying? depending on the market it definitely is up there. Most Challenging? It depends, there are definitely more challenging things than frontend, but in today's climate you definitely shouldn't bored. Career Prospects, job security? Job security is most likely, although if you do your job right you're digging into your own job security lol Career prospects are questionable, 90% of companies still don't have front end architects, and I'm not aware of many Principal engineers who come from frontend, so after Senior you're kinda stuck. Hopefully that'll change soon

Any response. I think it's a interesting career option but not the hottest. Backend developer have a lot to talk and with the rise up of DDD, CQRS, ES an some interesting backend architecture there are a lot of potential.

Frontend developer is an interesting career but not the hottest or the best, simply another interesting option :)