Given:
-these historical positional adjustments are accurate
-2.33 win replacement level adjustment
-0.380 win percentage for a replacement-level starting pitcher
-these historical league average wOBAs and scalings
-Edgar was -10 as as defensive third baseman
-Alex Rodriguez was +10 as a defensive shortstop
-Ken Griffey Jr. was +5 as as defensive center fielder
-Jay Buhner was +0 as a defensive corner outfielder
-Ichiro has been +0 as a defensive center fielder and +10 as a defensive right fielder
-a "full season" is any in which the player was expected to be a Major League regular, and includes seasons in which the player got injured
-1994 and 1995 prorated to 162-game seasons
Alex Rodriguez:
.402 wOBA as a Mariner, against a .334 weighted and park-adjusted league average. 40.3 WAR accumulated over seven seasons; 40.1 WAR accumulated over five full seasons, with 1996 standing out as his best. +8.0 wins per season when playing as a regular between 1996-2000. +8.0 WAR overall per 700 plate appearances.
Ken Griffey Jr:
.403 wOBA as a Mariner, against a .327 weighted and park-adjusted league average. 71.6 WAR accumulated over 11 seasons, all of them as a regular, with 1994 standing out as his best. +6.5 wins per season between 1989-1999. +7.2 WAR overall per 700 plate appearances.
Edgar Martinez:
.405 wOBA as a Mariner, against a .327 weighted and park-adjusted league average. 65.8 WAR accumulated over 18 seasons; 65.2 WAR accumulated over 15 full seasons, with 1995 standing out as his best. +4.3 wins per season when playing as a regular between 1990-2004. +4.8 WAR overall per 700 plate appearances.
Jay Buhner:
.371 wOBA as a Mariner, against a .328 weighted and park-adjusted league average. 34.9 WAR accumulated over 14 seasons; 31.2 WAR accumulated over ten full seasons, with 1994 standing out as his best. +3.1 wins per season when playing as a regular between 1991-2000. +4.0 WAR overall per 700 plate appearances. Jay got injured a lot.
Ichiro:
.350 wOBA as a Mariner, against a .324 weighted and park-adjusted league average. 37.5 WAR accumulated over eight seasons, all of them as a regular, with 2004 standing out as his best. +4.7 wins per season between 2001-2008. +4.4 WAR overall per 700 plate appearances.
Randy Johnson:
3.53 ERA as a Mariner, against a 4.51 weighted and park-adjusted league average (for starting pitchers). 50.6 WAR accumulated over ten seasons; 46.3 WAR accumulated over eight full seasons, with 1995 standing out as his best. +5.8 wins per season when pitching as a regular between 1990-1997. +5.9 WAR overall per 225 innings.
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Overall:
This isn't a perfect analysis. There's a lot in there with which you've every right to quibble. However, what's clear is that, when looking through this sort of lens, Alex Rodriguez is far and away the best Mariner of all time, and no little adjustments are going to make up the gap between him and the next player on the list. And that was with me being conservative on the defensive front. There's never been any evidence of Griffey having been a plus defender, and UZR called A-Rod a +13 defensive shortstop between 2000-2003, so if we change the numbers in the analysis to +0 for Griffey as a CF and +15 for A-Rod as an SS, then the gap grows even wider.
Even if you select each player's five best seasons (and keep in mind that A-Rod only had five full seasons in Seattle), you come out with the following:
Griffey: +41.4 WAR
A-Rod: +40.1
Randy: +37.3
Edgar: +32.2
Ichiro: +26.2
Buhner: +21.3
Five best individual seasons selected from Griffey/Randy/Edgar/Ichiro/Buhner overall pool: +45.1 WAR
Simply put, Alex Rodriguez is the best player in Mariners history. Now, whether or not he's the greatest player in Mariners history (assuming you have different definitions of "best" and "greatest") is up to how you weight things like character and team loyalty, but I'm not even going to try to go there. For purposes of this post, all I care about is establishing the fact that A-Rod is the best player ever to don a Mariner uniform. No matter what you think of him now, try to not let it tarnish your memories of his time in Seattle, because no star in team history has ever shined as consistently bright.
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As far as best individual seasons are concerned, I can't decide between A-Rod's 1996, A-Rod's 2000, and RJ's 1995.On the one hand, A-Rod OPS'd 1.045 and 1.026 as a plus defensive shortstop, but on the other, Randy posted a 2.48 ERA and a mind-boggling 2.08 FIP over 214.1 innings (in a shortened season). I'm inclined to give the nod to Randy, since he sustained his dominance into the playoffs and A-Rod's seasons were aided by inflated BABIPs, but then Randy's season was helped by an unsustainable home run rate, so I'm still stuck. I guess all that really matters is that all three seasons were unfathomably amazing, on the order of 10+ wins above replacement. According to Fangraphs, no one in baseball has posted a 10+ WAR season since Barry Bonds (and Adrian Beltre) in 2004.
The Mariners may not have any championship banners hanging from the roof, but for the better part of a decade, they were able to field some of the greatest talent in the universe. It's hindsight, it's hard not to feel a little bit blessed.
Fun fact: Randy's .384 OPS allowed against left-handed hitters in 1995 was half the league average.