For me overall was definitely Consort Radahn but man I loved that fight.

2 Freida from dark souls 3

3 The 2nd to last boss in LoP the electric armor chick.

One super easy boss that also took me 50+ tries was the first armor guy in flintlock I did him before any health buffs on hard and he was 1 shotting me and took a good 25+ parries to kill.