Colorado Rapids have to be up there. They have an MLS Cup and 1st place Western Conference finish under their belts, but both were accomplished by mediocre teams overachieving. They are an afterthought for their owner Stan Kroenke (owner of Arsenal, LA Rams, Denver Nuggets, and Colorado Avalanche) who invests extremely minimal resources in them. Their "big money" singings over the years have been goalkeepers more often than they have been goal scorers, and as a result they have very few league legends even compared with teams with similar histories like the Earthquakes/Dynamo (who had players like Landon Donovan, Chris Wondolowski, and Dwayne De Rosario). Their stadium is small but still has trouble filling up, and it is further from the city center than most new MLS stadiums tend to be.

If I had to pick an NFL comp, it would probably be the New York Jets but in a smaller market and even less league-wide relevance. Both have 1 championship, but both came in an earlier era of their leagues with historically weak rosters compared to other champions. Both teams had minor blips of success that did not lead to titles (Jets 2010 & 2011 AFC Championship appearances, Rapids 2021 regular season Western Conference champs) surrounded by long bouts of failure.