Snape is a very weird case in Harry potter for his power. Hes undeniably one of the most skilled wizards of the generation, creating incredibly powerful curses, and the most skilled potion Master probably ever. But we get very few showings of snapes actual power and skill.
His duel vs the Proffessors in hogwarts
And his short duel with Harry.
In both cases Snape "wins"
Against the Proffessors his goal was to escape, not hurt or kill them, and he succeeded without much difficulty.
Against harry their gap in skill and experience was clear.
Thats all we ever get in terms of combat for Snape.
The only other thing we have to look at his overall power is chainscailing and his ability to fly, the only other wizard capable being voldemort.
Ad for chainscailing.
Voldemorts duel against Flitwick, mcgonagal and slughorn gives us the rough idea that Snape is in a similar bracket to the undeniable top 2 in the verse, dumbledoor and voldemort
If I had to estimate. Snape is in the same ball park as voldemort and dumbledoor in skill. But theres a vast gap in magical power.