I agree with most of guys here,
I personally recommend these :
For Mainstream KPop style
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Narsha's NARSHA and
Ga-in's step 2/4 albums are highly recommended!

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CNBLUE, they're great as well
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Gummy, she's awesome. I recommend her latest Loveless album
-Solo tracks from Idol group members such as Taeyeon from SNSD (Can You Hear Me? from Beethoven Virus) and Yesung from Super Junior (It Has To Be You from Cinderella's Sister). They have great voices and sing good songs!
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Secret group also had a 'different' musical style from most Kpop idols nowadays
And it doesn't hurt to try the majority Kpop groups, especially the one who had been debuting for 2-3 years. Though maybe some people say that Kpop sounds the same, it's usually works for the promotional tracks. Digging depth in the albums and you'll find good materials in it
For indies
K-Indies has been one of my 'drugs' these days, if you want to see some variations from the usual Kpop, this is the genre imo
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LOVEHOLIC / LOVEHOLICS, band with many great songs and heartfelting ballads. Dramatic Cinematic alum would compile mostly everything they've worked.
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Clazziquai Project / Clazziquai, Electronica with bits of experimentations
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Humming Urban Stereo, Lounge music
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Casker, Lounge electronica sometimes with jazz and acoustics
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Epitone Project, this one have most music in instruments but his songs are great too, especially the one with Shim Gyuseon

I recommend Short Stories vol. 1 album and Lost and Storage album.
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Nell, an awesome band they have that kind of rock-ethereal-ballad combination. As far as I know this band makes their own music and lyric whose Vocalist, Keyboardist make BoA's One Star (Implode) on HURRICANE VENUS album.
Hope you'll like it