Okay I've been sitting on this for a while but for Great Merchants I think they should have some ability which relates to Corporations.
My suggestion is the ability to construct a super-franchise in another city. Which basically counts as 2 franchises in that city and increases your franchise max by 2.
I feel like towns scale very badly and the WLTKD bonus is rather weak as well but I guess you could keep it for civs who don't get a corporation.
Perhaps you should actually pop a great merchant to be able to found a corporation, sort of like how you spend a prophet to found a religion. So an extra "button" when you have the appropriate tech and monopoly. But it might not be technically possible to do it, or if one can tie it into as you found the town or if you can do the action in your own cities that is now the company HQ city.
Still the towns might be somewhat weak but they are ok and can create a usable workable tile out of nothing. The biggest benefit tho is the WLTKD that can reset the bonus resource you might not be able to get and there are just a handful of buildings that can reset it (circus etc) or triggering golden ages. So they do have their usage.
If one is more general about it tho a lot of them start out great but then turn out weaker as the game progresses but others are weak to start and grow better with age. Some are very specific to various games -- if you just do war then diplos are kinda bad, if you don't do culture then who cares about musicians etc.
Every one that builds a tile is in someway good (better in some places, playstyle, for certain civs then others).
Scientists are quite good, but in then eventually become pointless since the science comes from other sources.
Merchants have already been mentioned.
Engineers are great at the start but if you keep using them for instant wonder builds they eventually become bad cause they won't do a wonder anymore since they need to scale with their tile improvement to build the later wonders. So it's a path you have to pick and chose from.
Diplo/Prophet/Merchant all sort of fall in the same category. Great for some things, pointless for others. Such as if there are no more embassies available and you don't care about the statecraft game then they are almost pointless and certainly so vs the AI where one of them can't even instant flip a city-state since the AI have somehow built up enormous amounts of points due to them spamming out envoys etc.
You need a couple of prophets if you do religion stuff, if you don't then you need none. Merchant mentioned above.
Writers are great at the start when a small culture improvement makes a lot of difference but eventually their usefulness diminish as getting another single digit token for a city does nothing. So eventually you just try and use them to cut some turns for the next policy. Artists and Musicians are somewhat similar in that regard but with special actions.
General, probably the best or most important of the GP. Since it can change the world -- or the border of the land. I can't think of any game or victory condition where it is not important or great to have. The Aura, the citadel and naturally boosted even more if you get lebensraum that double the range in all directions of the bomb.
Admirals are similar aura vise if you use a fleet. Otherwise they are just "here have two copies of some random luxury and 2 points of unit cap". The cap should be increased, 2 might be a lot early but late game 2 is a pitiful amount.
Prophets have already been mentioned.
So in my mind they all have their usages, but the General is the supreme GP-unit way more important then the rest of them.