I originally referred to them as my "guilty pleasure", but since I'm going to see them tomorrow (and am pretty stoked, to be honest), I'm admitting it right now: I'm a fan of 5 Seconds of Summer.
The Australian guys has long been associated with One Direction after rising to international fame following their stint as the openers for the British boy band, but they're coming into their own with a handful of headlining tours under their belts. A lot of people in the pop punk scene can't stand 5SOS, but I don't understand the hate they get: while their music isn't anything groundbreaking, it's still good.
Plus they've worked with a variety of legit pop punkers, like members of Good Charlotte and All Time Low, which means they're slowly introducing their (mostly young and female) fans to pop punk bands that have been around for a while. Personally, I find that 5SOS can fit into just about any playlist I'm making, alongside bands I've been listening to for the past decade.
Say what you want about them, but they're hardworking and actually play their own instruments (which is what makes them more than just another "boy band"), and they're capable of writing some hella catchy tunes (even if they don't really think outside the box when it comes to lyrical content).
They're well on their way to creating their own distinct sound and if they follow their own advice ("don't stop doing what you're doing"), they'll be around for a while.