Boys and girls.

Lady Gaga. (Every single one of her contemporaries have stolen ideas from her.)

Stefani

EDIT: and Beyoncé as well.

Adele.
No one else.

ADELE. Everyone copying Rolling in the Church and Someone Like You to get a hit nowadays.

Adele.

ADELE and it seems JT/Bruno Mars (could lead to a more male pop era, but I doubt it)

But it will be interesting the next era from Lady Gaga of course.

There's only 2 original trendsetters on that list and those 2 are Britney Spears and Katy Perry.

We live in a music world of Britney wannabes and Jessie J.

Rihanna and Gaga.
kinda doubt Gaga's trendsetting feel, impact she did have but trendsetter its a 50/50

lady gaga

J-lo.

Britney, Rihanna and Adele.
Some on the lists haven't set trends, they simply copied things and I am not seeing how people have followed their lead.
Male dominance is apparent in 2013 but I'm not sure that constitutes as trendsetting.

No doubt that Britney did set trends some time ago but now?

All of the above.

Adele. The countless number of Someone Like You rip-offs charting and and even her throwback hairstyles and vintage clothing have been popping up more lately as well. (And the fact she - and other indie acts - took the industry from the Gaga era of weird clothing and alter egos, etc and brought it back to a more simpler level. She also killed the dance trend IMO, though it would have died at that time anyways tbh. People were sick of it with or without Adele.)

Rihanna is the other one as well but she doesn't really influence anyone when it comes to her music though. There's no denying Gaga had an impact when it came to setting trends but she's been away for awhile now and obviously she's not influencing things like she once was. She might when she comes back but we'll have to see.

Most people have done away with the "weird" look now though and it's no longer considered cool.

Madonna is a current trendsetter since she is inspiring popstars worldwide til this day

I love posts like these.
All it takes is one single song for insecure stans to assume someone is copying their favourite artist.

Last time I checked, Katy was a bubblegum/pure pop act who used to borderline on pop-rock in her debut. Jessie J's material is very diverse but mainly sticks to soul/funk/jazz influences (key word: influences). "Domino" is basically the only thing these two have in common. Educate yourselves, Katy Kats!

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As for trendsetting, all of them have had a helping hand at one point or another.

Rihanna is the biggest trendsetter in pop atm.

Rihanna and Katy Perry follow the trends.