Jack Russells. Have owned one and lived with others.
Love them, but while ours was great with the DC she would threaten to punch out the lights of any dog who thought it might get in her way, no matter how big it was. She never got into any fights, she never needed to, she'd just have GSDs backing away with their tails tucked (two of them at once, on one memorable occasion: 'I say you fuck off, so you fuck off, right?' 'Of course, madam, at once, are we fucking off fast enough...?').
And despite her general lack of leg length, she could easily keep up on a seven mile bike ride, and needed >1hr exercise a day. Hunted rabbits. Would have killed any rat that got in her way. If we let her, would stand on the window sill threatening vengeance upon any dog that walked past.
Since ours passed over to the Other Side (and I won't specify which Other Side this was), another Jack Russell has cropped in the neighbourhood. He read the manual: he is a little shit.
Working-line spaniels can also be, um, challenging. Just in a different way.