I think the impact an artist makes hugely determines their legacy in the future. With that being said...
Lady Gaga, Michael Jackson, and Madonna
All three are amazing in their own way, whether it be their actual compositions, fashion, or artistic values. Each of the three have impacted the music industry in such a way not many other artists have: they have been, or currently are, a
phenomenon.
Some of you said Lady Gaga isn't "amazing," but I think what's amazing about her is how fearless she is when it comes to expressing herself. Not only that, but her songs are actually
really catchy as well; she knows her way around a great hook.
The Beatles, Take That, Spice Girls
These three groups have gone on to show critics they can stand the test of time; the former two are without a doubt much more popular than the last, but no one can deny the phenomenon the Spice Girls caused worldwide their "Girl Power." I'm pretty sure The Beatles and Take That may be listened to for quite some time, I'm not entirely sure about the Spice Girls; I still believe they'll be appreciated to an extent, though.
Girls Aloud, Leona Lewis, Kelly Clarkson, Carrie Underwood
Living proof winning a TV talent competition isn't just a fluke, after all. They're all successful in their own countries, some of them internationally as well, but they've gone on to show musical critics they're to be taken rather seriously, and aren't as manufactured as people think they are.
There are a few more I'm considering, but I would want to wait a couple years before actually mentioning them.
The "TV Talent" group may seem sketchy, but considering each artist has a 5+ (or approaching one) career, I'm pretty sure they'll still be appreciated in the future, as well.