Although it says nothing about exact ranking, we can get a good idea of who should be included in this debate by looking at GPG since 1930-31 (the first full season of forward pass with offsides) with a minimum GP of 700:
Ranking-Name-GPG-Best Goal Finishes
1. Mike Bossy- 0.76 (1, 1, 2, 2, 2, 3, 3)
2. Mario Lemieux- 0.75 (1, 1, 1, 3, 3, 3)
3. Pavel Bure- 0.62 (1, 1, 1, 3)
4. Wayne Gretzky- 0.60 (1, 1, 1, 1, 1, 3)
5. Brett Hull- 0.58 (1, 1, 1, 2, 3)
6. Bobby Hull- 0.57 (1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3)
7. Rick Martin- 0.56 (2, 3)
8. Maurice Richard- 0.56 (1, 1, 1, 1, 1, 2, 2, 2, 3)
9. Phil Esposito- 0.56 (1, 1, 1, 1, 1, 1, 2)
10. Cam Neely- 0.54 (2, 3)
Although it of course tends to favor players with shorter careers, such a ranking eliminates so-called "compilers" who racked up high totals by playing a long time- a major problem when using goal/assist totals without any kind of context. Alas, numbers really do tell most of the story here; all but two players on this list led the league in goals at least once, something that should be a requirement in a discussion about the best in NHL history.*
As it is, the above list is rather diverse in terms of era and represents most of the players you'd likely include in a top ten. I myself would only drop Neely and Martin. Howe (1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3) of course deserves to be there, but is punished by GPG because his career was the longest. The other spot could be occupied by any of the following:
Alex Ovechkin 0.65 GPG (1, 1, 3, 3)
Teemu Selanne 0.51 GPG (1, 1, 1, 2, 3)
Guy Lafleur 0.50 GPG (1, 2, 2, 2, 3)
Charlie Conacher 0.49 GPG (1, 1, 1, 2)
Bernie Geoffrion 0.45 GPG (1, 1, 3, 3)
Now, where does this leave Iginla? Admittedly, he's on a very short list of players who have led the league in goals twice since 1930- in conjunction with the above 12 players, Peter Bondra, Bryan Hextall, Bill Cook, Jean Beliveau and Doug Bentley are the others (I think- I might be missing someone).
But just being one of them isn't good enough to be considered one of the ten best in history. Besides his two top seasons, he's only finished in the top 10 in goals one other time (2008, 3rd in the league). Even Bondra has a better resume than that. Is Iggy a better all-around player than Bondra? Sure, but that's not the debate. When discussing the ten best scorers in NHL history, Iginla falls short of the standard.
*- As an aside, I know many of the history folks here look at scoring finishes with Gretzky/Lemieux removed, the idea being that they were just so good that the players under them tend to get lost in the noise. It's a valid point, but not when it comes to a discussion like this.
Here are the years Gretzky or Lemieux led the NHL in goals, as well as the player who finished #2:
1982 (Gretzky) Mike Bossy (See above)
1983 (Gretzky) Lanny McDonald (Never finished top three again)
1984 (Gretzky) Michel Goulet (Never finished top three again)
1985 (Gretzky) Jari Kurri (Led NHL in goals in 1986)
1987 (Gretzky) Tim Kerr (Never finished top three again)
1988 (Lemieux) Craig Simpson (Never finished top three again)
1989 (Lemieux) Bernie Nicholls (Never finished top three again)
1996 (Lemieux) Jaromir Jagr
No single player was so consistently overshadowed by Lemieux/Gretzky that they would be in the top ten conversation had neither been born.