Ya'll...I'm literally 15 minutes into my first viewing of Boys Over Flowers and I have Second Lead Syndrome so bad.... This is definitely not good....

I'm curious to know what shows people suffered horribly from SLS...

Mine are Heirs and She Was Pretty!!

"She Was Pretty" - definitely!

- Reply 1997/1994/1988 - find out who the female lead's future husband is! You might suffer from SLS or not, depending on who you're rooting for.
- Angry Mom - though I wouldn't call the guy a second lead since it's not romance... more like "liking somebody you're not allowed to like".
Cheese in the Trap - one of the dramas that has made me suffer from a severe form of SLS. Don't watch it if you're weak but do still watch it for a rollercoaster of feelings!
- Cinderella and the Four Knights - somewhat similar to "Boys Over Flowers" but at the same time not? It got lots of lovey-dovey moments at least (+SLS). I liked it but I think it depends on the person - my friend didn't like it at all but that must be because of how different our tastes are, e.g. her favorite drama is "Playful Kiss" which I hated from the Earth to the moon, haha.
Drinking Solo - just a little bit of SLS but kind of like the same kind as in "Angry Mom".
Love in the Moonlight - I liked with whom the female lead ended up but I still feel bitter just thinking about the second male lead... for some reason.
Perseverance, Goo Hae Ra - for some reason the SLS in this one confused me. However, it was a nice 12-episode-long drama!

Those are just the ones from my list that I clearly remember or felt "bitter" about. Also, it's weird that SLS is such a horrible thing but at the same time I enjoy dramas a little more if it makes me suffer. :)
Who Are You - School 2015 was the worst case of SLS for me
I agree about the suffering with SLS!
Also, I did not finish Cheese in the Trap but Seo Kang Joon is my leading man when it comes to SLS! lol

I loved him in What Happens to my family, in Cheese in the Trap (What I saw of it) and his character made me melt in Cunning Single Lady.

Also, about you and your friends differing in opinions about dramas, I totally understand that!! A few people I've talked to HATED some of my favorite dramas:
ie: Emergency Couple, Angel Eyes, and Oh My Venus!

I'm going to check all of these out.

I have heard great things about hte Reply dramas, I've just never gotten around to watching them.  I'll admit, many of the dramas I've seen are romcom's, but I want to delve deeper into it and not just play it safe on shows I know I'll like.  My next one I think is going to be Two Weeks.  I've heard good things about it...and it has Kang Ha-nuel who is Drama Bae :)
MINOR SPOILERS
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I think my first serious case of SLS was in School 2015. Yup, I seriously thought the writer-nim would make those two end up together (you know who I'm talking about), but boy, was I wrong. That was really depressing...rip.

I second Reply 1988. Ahhhh, I was a Jung Hwan-Duk Seon shipper from the start, but converted to team Taek in the middle. I still felt really bad (esp that confession scene!!) for Jung Hwan, but I was happy how the drama ended (cue all the angry team Jung Hwan fans). AHHHHHH Hyeri and Ryu Jun Yeol are officially dating!!!!! Couldn't be happier. :-))))

I also had serious SLS in Dream High. Throughout the drama, it was hinted who Hyemi would end up with, but I was still surprised. In my mind, I kept telling myself, "It could have gone either wayyyyy." Yeah, that was pretty depressing...

Moonlover: Scarlet Heart. Need I say more? This SLS was a bit deceiving, since one of the guys started off really nice, then turned evil (classic). I shipped the right ship, but that ship didn't actually sail. 

Yeah, SLS has ruined my life. *cries*
Songyi...I have to agree.  That was a drama that I didn't complete, but Sung-jae's character had such a raw vulnerability to him that was SO hard not to fall in love with!!
totoro: I have heard soo many mixed feelings about Scarlet Heart.  Would you recommend it?  I want to watch it, but I also don't want my heart stomped on anymore than necessary :)
WHERE MY TAEBI SQUAD AT
Angela:
looooool! as soon as I read your reply, I immediately thought of 2PM's Hands Up!!
#taebi
Joy: ahhhh I would definitely recommend it. So many feels! It is melo, so be prepared. Yes, it's a historical, but it's definitely not boring. one of my all-time favorites. Check it out! ;)
Sassy Go Go: I felt so sad for Ha Joon (Ji Soo) cuz his friend ended up with his crush... :(

Angela: I really need Ji Soo to be a lead.  He's such an amazing actor and I just want to see him get the girl!!!
totoro: Well then...it's officially on my list! Thanks guys :)
YES, TAEBI.... That was one of the times I was most involved in the romance in a show at all. It was too frustrating!
KarenScott: is that a positive or negative thing? I can't tell lol.