CrucifiedRaven
The Evenflow Effect
I agree, but allow me to be more articulate.
If you're talking about "greatest tag team wrestler" as a wrestler who's best as part of a tag team, Billy Gunn really isn't that much to talk about on his own. He'd been given several solo pushes in his career, and all of them fell pretty flat. But what do people still remember him for today? The New Age Outlaws.
The Outlaws were, in my opinion, the greatest tag team in WWE history. Highly charismatic, excellent chemistry, but alone, neither guy was worth much. Road Dogg had some sweet Hardcore matches, but nobody was going to ever peg either one as a future HW champion.
And look back even further, to the Smoking Gunns. He and his brother Bart "Butterbean's Bitch" Gunn. They had a pretty powerful run in and of themselves.
And Billy and Chuck... Chuck Palumbo, another guy most people can take or leave, the two teamed up and turned what could have been a lower-card goofy homosexual gimmick into a bonafide media sensation that had them on... I think it was the Today Show?... accepting wedding presents AFTER they'd already taped the episode of SD where they turned heel, IIRC. They'd even gotten GLAAD on the WWE's side for all of three minutes.
Billy Gunn The One Ass Man really isn't worth much on his own at all. He's a generic power guy. But teamed with the right person, something magical can happen, and he's been able to hit on that magic three times.
I think it's ridiculous to judge a tag wrestler on their ability to be a solo wrestler. The question is who's the best tag wrestler, not who's the best solo wrestler than came from a tag team. That's why Shawn Michaels, Bret Hart, Chris Benoit etc shouldn't be counted in this discussion.
Top tag team guys who would eventually become top solo guys like Booker T and Edge count because their time in the tags could even outshine what they did in their solo career, though that is debatable.
Billy Gunn is no doubt at the top of this list, especially in the WWF/E, for all the reasons you mentioned above. But the fact his solo career didn't work out isn't relevant.