To give you an actual answer to your question, I think "Return of the Mack" is among my favorites. "I'm Gonna Be (500 Miles)" is also high on my list in terms of song quality, though I know The Proclaimers hace more than one hit in other regions.

The main reason I wanted to reply wasn't actually to give my answer, but to let you know that there's a whole YouTube series about one-hit wonders called One Hit Wonderland by the creator Todd in the Shadows. I'm not affiliated with the channel or anything; I've just found his breakdown of the pre-hit and post-hit careers, plus commentary on the song itself, to be an interesting lens through which to learn more about pop music history.