It also comes down to each particular weapons move sets. As well as whether you want quick hitting or hard hitting or something in between. While the Greatsword (which happens to be an Ultra Greatsword, go figure), for example, is the highest damaging weapon in the game, it isn't for everyone. The starting assassin weapon, the Estoc, is considered by many to be extremely OP and cheesy for PvP, but it's still not everyone's favorite weapon to use in PvP. Hell, Havel's Greatshield when 2 handed is an amazingly effective PvP weapon, but fails miserably as a PvE weapon.
Every single weapon in DS3 is viable. It just comes down to what you want to use (looks, style, move set, whatever), do you feel comfortable using it, and what do you want to use it for (PvP or PvE).
I think the problem with many of the boss weapons is that they are sometimes very situational. As some are extremely powerful versus certain other bosses, but might not be as good in general use. And there are always exceptions. For example, the Dragonslayer Greatshield, almost makes the Nameless King's second phase trivial. And that's just one of many examples.
And that doesn't mean that boss weapons can't be as good in general use as regular weapons. Like the above example of the Estoc being extremely OP for a quality PvP build. But the Crystal Sage's Rapier is equally as OP with an Int/Dex build and has the added bonus of +50 item find if you are trying to farm something.
And hell for souls fashion? Boss weapons are fantastic. It's hard to beat something as flashy as the Dancer's Enchanted Sword (used to be semi OP in PvP with endless stunlock but got nerfed) or the Demon's Fist (semi decent in PvP also as it gives hyper armor with the buff combined with quick parry animation).
Regards,
SB