Regular floor fights can usually be cleared without taking any damage even if you're "missing" a relic so Slaver's Collar seems like it's rather easily the best energy Relic. I'm surprised it's not "1 Energy each turn against bosses".
Not sure I agree with this. Act 2 especially has a number of fairly tough regular fights, like Snake Plant (8x3 damage constantly can be hard to keep up with, especially as you'll be frail for most of the fight) and both Chosen + other enemy fights (Hex is really tricky to deal with - get flooded with dazed, or stop playing skills which includes most of your block and tank damage, or somewhere awkward in between). Act 3 a bit less so, although the Maw and Spire Growth I do find somewhat tricky at times. And Darklings or Jaw Worms can sometimes just all attack at once and be threatening 50+ damage on a turn, which sometimes is dangerous.
You definitely take less damage on average from the regular encounters, but you also do a lot more of them. Having only 3 energy instead of 4 in the majority of fights, even if they're somewhat easier ones, can lead to you taking a fair bit more damage in those random encounters. Still overall I do agree it's probably one of the stronger boss relics, but I don't think it's easily the best.
Edit: Just lost a run in act 3, having taken about 50-60 damage in regular encounters, before getting finished off in an Elite fight. Granted my deck was kinda bleh and I didn't pick up too many good relics, but really, I just want to highlight that no, you don't get through regular fights damage free every time. Even watching the youtuber Jorbs, who seems very skilled at the game (pretty good win rate on ascension 20), he's often taking a lot of damage from those regular encounters.