Definitely listen to Day6. Whoever recommended that is a smart person.
SHINee's my favorite group, but if you watch their older stuff you'll see that there are five members, but there are currently four members because Jonghyun passed away in December.
EXID, Red Velvet, Winner, Pentagon, EXO, BAP (their music videos are incredible especially One Shot and Skydive, but 1004 (Angel) is probably my favorite song from them), and Mamamoo (these girls have really good voices)
I honestly could list off more, but the further you get into Kpop, the more you'll discover on your own, but if you want songs for each group to listen to, then let me know.
I don't really know what your style of music is, so I'm just going to recommend the songs that I and/or other people tend to like to get you started:
Day6- I Need Somebody (it's a lyric video, but I personally think it's their best song) and Shoot Me is what they recently put out as a music video. Honestly, all of their songs are really good, but I recommend watching with the subtitles on so you can feel the pain.
BAP- if you want a more upbeat song then listen to Honeymoon or Feel So Good (the dance practice is also good), but BAP is known for more of their darker/gang related theme in music videos in which case watch Skydive or One Shot (both MVs are pretty long (Skydive's 10 minutes) but they are honestly so good and well produced that it's like watching a movie) and again, 1004 (Angel) is my favorite from them. However, with BAP, their company is crap and they mistreat their artists (BAP filed a lawsuit against them to try to get out of their contracts and that went on for about a year before they settled and had to stay in the company until their contracts expire) and one of the member's contract expires in a couple months and the last one expires in I think January and I don't think they're going to resign with that company and there is a possibility that they will disband, so just keep that in mind, but still check out their music because it's really good and the members are all great
EXID- Ah Yeah
Red Velvet- Bad Boy
Winner- Really Really
EXO- Love Me Right or Growl
Mamamoo- Starry Night
4Minute (they're disbanded, but one member (Hyuna) still makes music as a solo artist)- Canvas and Hate
Wonder Girls (disbanded too)- Like This
Girl's Generation- You Think
NCT 127- Limitless
Monsta X- Beautiful and Shine Forever
BtoB- I'll Be Your Man
BTS- Spring Day and Save Me (also, check out Let Me Know; there's no MV, so you'd have to watch a lyric video)