Tibetan Spaniels are on the short list of breeds with a long, long history of being bred for their alert barking. I don't have much first hand experience with the breed, so I don't know much other than their watchdog heritage and the fact that they're supposed to be quite sensitive and do best in low-key households without too much shouting and chaos.
My poodle (oversized miniature, probably with some standard heritage, maybe with some toy) has always been a very astute alert barker. He's always very quick to alert to unusual noises or events, but adjusts quite well to 'normal' sounds and doesn't go off every time the neighbor starts his lawnmower or the landlord opens his garage. He cannot have access to a street-facing window or he will scream at passing dogs, but that's more of a him issue than related to his breed. My younger dog (Lagotto Romagnolo) takes his alert barking duties VERY seriously but unfortunately also thinks we need to know if someone got up too quickly and bumped their chair too loud, so I've come to appreciate my poodle's discernment a lot more lately.
My in-laws' late standard wirehaired dachshund was also an excellent watchdog (though to note, many wirehaired lines - at least here in Norway - are still bred for hunting and can be more intense than other dachshund types). She wasn't one to actually do a lot of barking herself. If there was another dog around, she'd just give a quiet "wuff" and let the others lose their heads barking at the thing she'd alerted them to. I'm not sure how she'd alert with no other dogs in the household - when we visit there's always our dogs or my MiL's Leonberger around. But she was also very judicious about her alerts and did not like to waste energy barking about something inconsequential. ...sometimes she faked an alert bark when the other dogs had something she wanted, though, so they'd run off and she'd steal their chew for herself.
There's probably other breeds that would be excellent choices, but I thought I'd highlight the ones I had personal experience living with and the one I know was specifically bred for watchdog traits.