Mogilny > Joey Mullen. Both offensive minded RW's, but Mogilny had a better PPG for his career and higher peak value. Mogilny's greatest accomplishment is a 76 goal season. Mullen's is being the first American 1000pt man. Not sure who was first, but Mogilny and Fedorov were pretty close to being the first Russian 1000 pt men.
But take a deeper look. Mullen had seven 40-goal seasons. Mogilny had three 40-goal seasons, and a 39-goal season. You can cite era all you want, but Mogilny's biggest problem was motivation, not the era. He had the talent to score 50 goals a year post 1995-96. But he rarely used it.
Mullen won three Cups. Mogilny won one. Mullen was a first-team all-star the year the Flames won the Cup. He was an important addition for the Pens the year they won their first Cup, provided pretty good leadership. In their second Cup, he was injured most of the post-season. He also tied for the Flames playoff scoring lead when they reached the final in 1986.
Mogilny? Well, he won a Cup in 2000 with New Jersey, but was he a key player? No. If Mogilny only gets seven points in the post-season, you know he wasn't a key player, because he doesn't give you much defensive play, leadership or grit. He went goalless for about a month when the Devils went to the Cup final in 2001. He's over a point-per-game for his career in the regular season, but he scores at under a 60-point pace in the postseason.
Sergei Fedorov belongs in the HHOF. Mogilny does not. Fedorov was a key player on three Cup champions, and won a Hart and two Selkes. That's good enough for me.