I'll go by bowlers who have played 50 tests.
Garner, Ambrose, Marshall all averaged between 20 & 21. Marshall had the best strike rate out of the three, Ambrose had the best economy out of the three, Garner was in between, Outstanding.
Statistically, the next best fast bowlers to have played 50 tests are Glenn McGrath & Fred Trueman, followed by Allan Donald & Sir Richard Hadlee.
I was looking at Waqar's stats, he was only ever a really genuinely threatening bowler in the 4 early years of his career. Excluding his poor first 5 tests for this analysis.
From his 6th test to his 32nd test, precisely from October 10 1990 to October 2 1994, he was extremely deadly, 19 five-fors, 4 ten wicket matches. In this time he averaged 17 with the ball, with a phenomenal strike rate of 32.7, & an economy of 3.17 (below his overall career economy).
In the rest of his career (55 matches) he only managed 3 more five-fors, & 1 more ten wicket match. In this time he averaged 28 with the ball, 2 of the 3 five-fors he took were against Zimbabwe & Bangladesh, & the 3rd one was part of his only ten wicket haul, againt South Africa in 97/98.