That's a lot harder one than most frequently cast spell, imo.
To NPCs I'm going to guess 1 because many, many characters can hit for that, some NPCs have damage reduction from super high AC or something that causes very small hits, spells can partially resist down to 1, slam always hits for 1, offhand swings will more frequently hit that low without a damage bonus, etc. Also disease cloud ticks for 1 if we're counting dot damage.
To players I really have no clue because of how mob damage/kicks/bashes all scale and damage interval is in play. For the sake of throwing some guess out though, I'd guess something along the lines of 8, 10, 12, even maybe 20? Low-level mobs can max that hard, mid- and even high-level mobs can get minimum hits that low, lots of bashes and kicks can hit for those amounts, etc. I don't think it will be 1 because many mobs cannot even hit that low (it's hard to find reading material on damage bonus+damage interval for NPC melee damage calculations,
https://wiki.project1999.com/Sakurag...de#Disciplines or threads about when warriors should use evasive or defensive get into it some (
https://www.project1999.com/forums/s...3&postcount=15), but suffice it to say that mobs above a certain level are incapable of hitting as low as 1 damage), and the mobs that can hit that low are usually going to miss higher-level players they're swinging at anyway.
edit - in my shaman's logs of levels 1-49, the lowest level monster to hit me for only 1 damage was a wisp.