The worst NPC has to be someone with the potential to be a real NPC with a background and a story, but who is never given the chance to share it. The Wizard and Mr. Qi aren't really fleshed out much in excess of your typical zombie but they are still key parts of Stardew Valley, though not the most vibrant of personalities. They both make up for that with style-- nice saving throw!
But who is in most desperate need of a backstory fill? Sandy has a small backstory and you get hints of her from Emily pretty early in the game. The Old Mariner-- hmm, you know, he really is the weirdest of the bunch. A guy who shows up only on rainy days, selling some 21st century snake oil aphrodisiac that reprograms the minds of young single people you've plied with gifts into settling down on a farm in lieu of Gridball fame or a Nobel prize. That's *magic*! I wonder what he does on sunny days? I assume he fishes for Mermaids, but what do you use for bait?
No, no, it's The Henchman who is in most dire need of some NPC love. A lone sentinel barring your way, he's there to save you from yourself. Standing all alone in a murky swamp for who knows how long, charged with maintaining the Farmer's integrity, he is nevertheless the NPC most rewarded by the Stardew Valley speedrunners out there, released from his bondage with barely a couple of lines before he's whisked away to who knows where. What was his previous job and did they recommend him for this position? Did he end up in a nice house in The Hamptons? Did he inherit that cleft chin from Roger Moore?
But perhaps there is moore to The Henchman's life that's only been hinted at. After all his employer provides more than just a paycheck, she also uses knows his favorite treat and bestows upon the Farmer the means to make it for him. Quite thoughtful, really. Though with many such superficial gifts comes the temptation to follow, and how many Farmers have resisted those various misdeeds?
The Desert Obelisk is nice, though. Better yet, hopefully the 1.5 update includes a Pink and Chocolate Cake Dessert Obelisk. Mmmmmmm!