No regrets:
Swimsuit event part 2 gacha. Artoria and Marie are amazing.
Accel Zero Order gacha, I didn't get Kerry, Iskandar or Waver but I don't regret trying.
Aniversary Okeanos gacha, I had every rate up servant other than Medea Lily (which I was aiming for) and I got her, no dupes on the way.
Prisma⭐Illya gacha, I got every rate up servant.
Halloween 2016 gacha, got Cleo.
I regret everyrhing section:
First Gil gacha, I had no clue what "limited gacha" even was, neither a notion of that I had to hoard quartz for the future (first mobage experiencie). Didn't get Gil.
First Nero gacha, same as above. Didn't get Nero.
Orion gacha, same as above. Didn't get Orion.
First Kintoki gacha, didn't want him, just Saber Alter, but she didn't appear.
First Halloween 2015 gacha, I didn't actually want Tamamo but the hype of the rest got to me. No Tamamo to show for, gratefully.
Saber Wars gacha, I wanted none of the servants rated up, I wanted the 5* CE and I blindly dumped all of my quartz. 0 gold servants, no 5* CE.
Kara no Kyoukai gacha, really wanted Void Shiki, but I had her niche covered already and she turned out not-so-good. Dumped a good amount of quartz, no Void Shiki for me.
First Amakusa gacha, I wanted a ruler. He's freaking bad, glad I didn't get him actually but ouch my quartz.
Jeanne Alter second gacha, I kinda wanted her, now I don't. She ate up my quartz, leaved me nothing meaningful and everyone and their grandma has Jalter on All slot lol.
Karna&Arjuna first rate up gacha, wanted Karna, dumped two tenrolls, got 3* shit.
Shuten gacha, I didn't throw much quartz into this, but every single ticket or yolo that went her way was a mistake, never roll for CEs seriously lol. No Shuten thankfully.
First Scathach gacha, I wanted her, now I don't, I want that tenroll back lol. Lancer slot is full of Scathachs... and I didn't want her thaaat much but fingers are quick.
Swimsuit event part 1 gacha, awesome servants, I got everyone except Tamamo, but I actually just wanted Anne&Mary and now I feel bad for Kiyo and Mordred because I neglect them and they are good... but yeah... not of my liking. And I throwed in too much quartz.
America gacha, got nothing to show for, wanted everything but Nightingale.
In the end I conclude I regret rolling when I don't get the servant (wow what a twist) but I also roll too much and very randomly... I should check up my gacha addiction indeed.
(Every gacha not mentioned, I either didn't roll at all or rolled too little to care)