Tough question because I think these are the four best SPIKE games.
Toss up between BK and DP for me. For a 1-game setup I might err on the side of DP simply because the code is much further evolved than BK; that being said, BK code is getting ongoing updates and looks as if it could mature into something truly great. Both games are gorgeous (unless you dislike red!) and have great themes, and both shoot very well and have a nice variety of shots and classic pinball goodness like spinners and drop targets.
IMDN and SW are great too, but to me both fall a little short of the “total package” you would want in a single-pin situation.