I personally take this with a large grain of salt, but according to one London-based NGO (SNHR), which appears to have one of the most detailed attempts at accounting
The report reveals that no fewer than 230,224 civilians, including 30,007 children and 16,319 women (adult female), were killed at the hands of the parties to the conflict and controlling forces in Syria between March 2011 and March 2023. The Syrian regime was responsible for 201,055 of these deaths, with the victims including 22,981 children and 11,976 women, while Russian forces killed 6,950 civilians, including 2,048 children and 977 women. Meanwhile, ISIS killed 5,054 civilians, including 958 children and 587 women, while Hay’at Tahrir al-Sham (HTS) killed 517 civilians, including 74 children and 79 women, and the Turkistan Islamic Party killed four civilians. All armed opposition factions/Syrian National Army (SNA) were responsible for the deaths of 4,206 civilians, including 1,007 children and 885 women, while Syrian Democratic Forces (SDF) killed 1,420 civilians, including 250 children and 169 women. Additionally, international coalition forces killed 3,051 civilians, including 926 children and 658 women, and finally 7,967 civilians, including 1,763 children and 988 women, were killed by other parties. The report also includes a running count of the civilian death toll and its distribution by year over the past 12 years. Analysis of the data shows that 91 percent of all victims were killed at the hands of Syrian regime forces and their allies, with roughly 56 percent of all victims being killed in the second and third year of the popular uprising (between March 2012 and March 2014).
I suspect Syria and Russia disagree with some of this, but I could not quickly find any counter-figures from them.
One reason I'm skeptical is that the same NGO e.g. said:
“The air force [is responsible for] approximately 70 percent of these deaths. Missiles, barrel bombs, lead to massive destruction and lead to massive casualties. And only two parties have an air force - the Syrian regime and the Russians. Also the US-led coalition, but they only operate in very specific areas.”
I mean Syria had their own air force, but did they kill 28.5 times as many civilians as Russia's air force, when the Syrian one was largely reported to be on their last legs and only saved by Russia intervening? Even with that 0.7 reduction factor applied only to the regime's deaths, that's still a 20x factor over Russia's air force. So, YMMV. Yeah, it's possible Russia had more precision ammo.
A RAND report in contrast claimed (previous link)
"negating Syrian air power would have only a marginal direct effect on civilian casualties, which have mostly been caused by ground forces".
Anyhow, another opposition-friendly group, the SOHR likewise claim that 87% of the civilian deaths were caused by the regime. From their figures, I'm skeptical of the combatant to civilian death ratio e.g. 8:3 claimed in 2014, given the largely urban nature of the combat.
BTW, the UK government claims it killed or injured around 4400 ISIS militants and most of these are claimed as fatalities although just about 1000 of them in Syria (and 3000 in Iraq) but only one civilian killed in Syria+Iraq between 2014 and 2021. That alas can't be easily cross-checked with much else, because even SNHR data doesn't break down "coalition" strikes by nationality.
More broadly, the Western coalition in this war seems to have admitted to only about 10% of the civilian deaths that NGOs (like Amnesty) accused it of. I'm guessing the Syrian regime and Russia generally didn't [bother to] admit to any (numbers in particular).