For me it's the Summoner (surprise!), followed oddly enough by the Legacy, of which I own five (seems like every time I used to get an Ebay lot, it had a Legacy in it!).
I never did like the Shootist, like a half-assed attempt to fix the Hunchback. You're still 4/6 with an AC-20 as your primary weapon, even in an era of XL-engine heavies able to hit 5/8 (see: Flashman, Lancelot). So you're either never able to bring the gun to bear, or you're using it on similarly-slow units that probably outgun you. Factor in that it always was kind of boring looking to me in art and pewter... not one I'll be missing if it goes to the archive.
Shootist was one that I just liked the art for when I first saw it and learned to like and use the stats for it. No, it's not amazing, but before you had things like royals running around, I wouldn't mind having one intercept a Flashman or Lancelot. Or team up with something else slow and steady like a Black Knight and zombie rush something larger.
Granted, I've only used them a few times as the whole rarity thing hinders that! It's also pretty solid in quick strike, being a standard engine, max armor, and a pretty good level of firepower if memory serves. I have my one so even if it archives, it's not a huge deal to me I guess.
As a caveat, I'd love for every miniature to be available without an archive fee, don't get me wrong. But I understand that as a business, IWM has conserve space, manage it's few employee's time, and allow for new releases. This list is sad, as I like the Falconer, Specter, Talon, Shootist and a few others, but I understand that they aren't selling in a great enough volume to justify their easy access. :(