sittler was before my time so i can't really say much about his game. but i tend to see sittler in a group of 70s and 80s generic superstar scoring centers who over fifteen years from the early 70s to the late 80s took turns having career years where they would snag a second team all-star and sneak into the MVP conversation for a year.
so you have ratelle, perreault, sittler, stastny, savard, and hawerchuk. i think clearly espo, clarke, dionne, trottier, mario, and obviously gretzky were a class above.
of the guys i named, sittler has the largest gap between his career year and his second best year.
his big year was 1978, when he scored 117 points, finished 3rd in both the scoring race and for the hart trophy, and got his one second team all-star (behind trottier's breakout year). sittler has four other top 10 years, but they are all 8th or 9th places. his career year is 17 points higher than his second highest scoring year.
whereas ratelle has his big year in 1972. 3rd in scoring and MVP, 109 points. over a decade-long prime, he has seven top 10, mostly in the 6-7 range, finishing as high as 4th in the much lower scoring 1968. on raw numbers, ratelle's second best year was 105 points (6th), though it should be noted that not only was that a higher scoring season (five 110+ scorers that year, vs just espo/orr), ratelle also missed 17 games in 1972 and his points/game was an astonishing 1.73, only 0.02 behind espo for number one.
perreault's big year was 1976. 113 points, 3rd in the league, one of back-to-back second team all-stars (both behind clarke, then dionne), though unlike everyone else on this list he got almost no hart consideration that year. his next highest scoring season in raw points was 106 points, good for 4th, and has five top 10s overall, three of them in the top 5.
stastny has a great run of three years, all with 120+ points, but his best one was 1982, when he scored 139 points, 3rd in the league, 4th for the MVP, third in all-star voting behind gretzky and trottier. stastny has the best prime of these players, with six top 10s over eight seasons, but all are top 6, and he finished as high as 2nd.
savard had two peak years, 1983 and 1988. in '83, he scored 121 points, finished 3rd in scoring, 3rd for the MVP, second team all-star behind gretzky. in '88, he had a numerically greater year, 131 points, 3rd in scoring again, 5th for the MVP, third team all-star (behind mario's first MVP season and gretzky). in the same prime years as stastny, savard had five top 10s, the two 3rd places and the rest in the 6-7 range.
and finally hawerchuk in 1985: 130 points, 3rd in scoring, 2nd in hart voting, second team all-star. he only had four top 10s, but his other super high one was 121 points (4th).
then in 1989, yzerman has his 155 year and raises the ceiling on the generic superstar scorer.
anyway, insofar as we see these guys ahead of the one year wonders like larouche, nilsson, and maruk, who don't have a handful of other top 10s to back it up, or federko who didn't have the one spike season, i feel like sittler is the lowest of the group, with stastny being the highest.