this is in no particular order, and these are my personal opinions. let's see if we have the same taste!

SHINee:

• Hold Me
• Alive
• View
• Odd Eye
• Trigger
• Farewell My Love

OST:

Jonghyun:

• 산하엽 (Skeleton Flower/ Diphylleia Grayi)
• End Of A Day
• Crazy
• 그래도 되지 않아? (Fine)

Taemin:

• Pretty Boy (ft. Kai of EXO)
• Experience

Infinite:

• Love Of My Life
• 발걸음(Walk To Remember)
• 엔딩을 부탁해 (Up To You)
• 24時間 (24 Hours)
• Can’t Get Over You

Sungkyu:

• 너여야만 해 (The Answer)
• Kontrol
• Alive

BTS:

• I Need U
• 쩔어 (Dope/Sick)
• Outro : Love is not Over
• For You
• Let Me Know – Japanese Version
• Outro: House Of Cards
• Run
• Butterfly
• Ma City
• 고엽 (Fallen Leaves/Dead Leaves)

Solo:

• Jin : I Love You ( Cover | MATE: I Love You)
• Jungkook: Lost Stars (Cover | Adam Levine: Lost Stars)

​(okay i'm a literal BTS trash now)

Red Velvet:

• Wish Tree
• Dumb Dumb
• Day 1
• Huff n Puff
• Ice Cream Cake
• Automatic
• 사탕 (Candy)

OST:

• Return (ft. Yuk Ji Dam | Who Are You: School 2015 OST)

f(x):

 (ok i didn't get any gif of them as four ;;)

• Diamond
• Four Walls

Amber:

• Shake That Brass
• I Just Wanna (ft. Eric Nam)
• Beautiful
• Heights
• Love Run

SNSD:

• Catch Me If You Can
• Lion Heart
• You think
• Party

Taeyeon:

 omg she's a godess for real T~T

• I (ft. Verbal Jint)
• 스트레스 (Stress)
• U R
• 먼저 말해줘 (Farewell)

EXO:

 sorry i cannot find a full gif huhuhuu

• Sing For You
• Promise
• Love Me Right
• Lightsaber (STAR WARS Collaboration)

Others:

• ChoA (AOA): Flame
• K.Will: Cold (Remember: War Of The Sons OST)
• DAY6: Colors, Congratulations, 이상하게 계속 이래 (Out of My Mind)
• Eric Nam: I’m OK
• Hanbyul: Shooting Star
• Kang Seungyoon & Sandara Park: The Two Of Us (We Broke Up OST)
• Yook Sungjae: Love Song (ft. Park Hye Su | Who Are You: School 2015 OST)
• Big Bang: Loser, If You
• TVXQ!: Rise As One (Changmin Solo)
• GOT7: IF You Do, Just Right, 고백송 (Confession Song)
• VIXX: Chained Up
• Ladies’ Code: 아파도 웃을래 (I’ll Smile Even if it Hurts)
• Ailee: Letting Go, Insane

WHEW! that took some time... /wipes huge drop of sweat/

anyways, here's my fav songs of 2015! actually, i went back to old songs A LOT and i had to wikipedia the group every time to see if it was really 2015 or not xD