I would choose the Tax Collector.
-When you look at the Wiki, it says that he can store a max of 33 gold, 33 silver, and 33 copper (max happiness), which seems good, until you do the math and realize that it takes him a WHOPPING 4.2 REAL-WORLD HOURS to collect it all.
-Even if the Tax Collector is unlocked as soon as possible in Hardmode, chances are the player has already set up a decent mob farm and could make 33 gold in roughly 30 minutes, depending on the speed of the farm.
-The Tax Collector does drop the Classy Cane, which is arguably one of the WORST WEAPONS IN HARDMODE. The thing doesn't even drop 100% of the time!!
-The Tax Collector is very rude to the player and quite possibly other NPCs
-No other NPC likes the Tax Collector so you can't use him for an NPC happiness pylon system
Conclusion: the Tax Collector is an absolute waste of an NPC slot.