Career - probably Gretzky, but there`s an argument for Howe.
Prime - I`d rank Howe ahead. Maybe you could argue for Bourque ahead as well.
Peak - there`s a solid argument for Orr.
Playoffs - no worse than 3rd behind Richard and/or Roy.
International tournament - probably an argument for Fetisov or Makarov.
If you look at it like that, Gretzky is probably no worse than 3rd in any category, and he has a legitimate argument for 1st in all five. Ranking Gretzky anywhere other than first isn't impossible, but it's a tough sell.
Prime - how is it not Gretzky? To me prime is the player's best year (minimum of 3, usually 5+) where he was at or near his best level. Important factors are heights of prime, consistency, and performance vs peers. Longevity also.
During his 11 first years, he won 9 harts, 9 ross, and 2 smythes. 8 straight harts. The one year he didn't win a hart/ross - he won a smythe. He also captained his team to 4 cups - and made players around him better (and in that aspect, he may also be #1 all time, though that's more subjective).
In what possible world does Bourque stack up? You could maybe say Bourque achieved a very very high level of play (Norris or close) - and remained consistent at that level longer than Gretzky maintained his level. So Longevity of prime = Bourque. But if you factor in height of prime and performance vs peer Gretzky blows him out of water. If you include playoffs in prime (and we should) - it's even a bigger gap.
I'm not sure off hand what you consider Howe's prime. His career is so long it's tricky. Obviously he can have Gretzky beat on longevity if you count his prime till age 41 and his 15th top 7 hart nomination in a row. Consistency and longevity is staggering. But performance vs peers Gretzky remains a huge step ahead. He torched the league in every way possible, was so above everyone for those 11 years (minus Lemieux near end). I don't think the longer longevity of Howe counters that. To me prime is definitely Gretzky. He was the clear cut best player in the world for almost 11 years straight (minus Lemieux). Was Howe even the clear cut best player for 4 years straight? At any point after that?
Peak. You might say there's a solid argument for Orr. I said that myself. I personally find there's an even stronger case for Gretzky. Consistency/length/playoffs being factored into peak i think gives Gretzky the edge, even if you somehow want to say for 1-2 specific season Orr might have been the better contributor on an overall per game basis.
Career. I mean to me, career is prime, peak, + accomplishments (team + individual). Accomplishments - Gretzky owns more records, trophies (individual and not) than any other player. his prime/peak i just spoke of. I have trouble not giving it to Gretzky easily.
Playoffs - we just did the playoff project last year, comparing playoff performers. Frankly I don't see the case for Richard over Gretzky AT ALL. Roy - maybe. A lot of that hinges on it being apples to oranges (goalie vs forward). I personally lean heavily towards Gretzky (3 smythes is awesome, but i feel as though Gretzky should have 4 minimum) - but ok top 2 at minimum, with a much easier case at #1.
International tournament. I also don't know enough about Soviet players to comment about that comparison. Suffice it to say Gretzky blows Howe/Orr/Lemieux away in international competition. So none of them make up ground on him here - he just distances them further.
I realize you're not specifically saying you rank Gretzky behind in all those categories you laid out - you're just opening up the argument that it's plausible. I still think it's not really all that plausible at all.
I don't see a case for Gretzky at #2.
Orr's playoffs are very bad (compared to Gretzky).
International and overall career is also lacking.
Even if you give the slightest of edges to him in prime/peak - Gretzky should destroy him thanks to the other elements.
Howe - very lacking on Gretzky in playoffs and peak (especially length of peak, but also height of peak i think). International tournament too i think (not sure?). I spoke about prime above, and also career. Even if you want to give him the edge in some aspects (and it takes a lot of work to get there) - Gretzky comfortable edges him out overall.
Lemieux - I love playing the "what if" game and speaking about which records Lemieux could have topped under better circumstances. But in an all time ranking there's never been a case to rank Lemieux ahead.
Yeah - I just don't see Gretzky at anywhere but #1 making sense. His resume lacks nothing.