I'm the same as you, only played the newest game (although I've played the fuck out of it) and I like Triss better

Triss seems to be a genuinely good person. She risks her life and gets tortured for Geralt and Ciri by telling Geralt to turn her in to the witch hunters, and she risks her life trying to get the mages out of Novigrad. I don't know how Yen would act in the first situation, but I guarantee she would leave Novigrad alone in the night, leaving the other mages to fend for themselves.

Yen is too high-and-mighty. I don't know how strongly others feel about that, but I absolutely cannot stand that attitude. Not just with her, with anyone I meet in real life. You know nothing about this person, yet it's perfectly acceptable to you to degrade them and act like you're life is more valuable than theirs? To me, anyone who feels the need to do that has some serious insecurities.

Triss also seems more realistic. I can't put my finger on exactly what it is, but Yen strikes me as kind of phony, whereas Triss speaks and reacts like you could expect someone too.

Last, I personally find Triss more attractive, don't get me wrong, Yen is hot, but I've always had a thing for gingers. Her outfits are more revealing, and she's got better...assets than Yen.

Obviously there's a lot of backstory with Yen, and Triss came in more recently as a fling, but to an uninformed player it was a super easy decision to make.