I'll be honest, I really hate love trinagles in my dramas, but by mistake, I did watch some love triangles.
Kill me, Heal Me (He's in a love triangle with his own alter-ego)
Another Miss Oh (She's in a love triangle because she has the same name as the girl the male lead loves)
Bride of Century (She's pretending to be the fiancee of the male lead, while in reality, that fiancee is someone else)
Mask (Here again, she's pretending to be someone else)
Smile, you (He's in love with her sister first, but later falls in love with her, I know it sounds too cliche but if you liked the love triangle of The secret life of my secretary as much as I did, chances are you'll definitely like this one too!)
Which Star are you from? (Male lead falls in love with a girl who looks same as his girlfriend who dies in an accident)
City Hunter (Same as Healer)
For love sqaures (Birth of Beauty, Prime Minister and I, Warm and Cozy, Ugly Alert)