People keep saying "basics" are the most important kata, essentially. I'd like to mix it up a bit, and say that I disagree, at least in semantics. Basics are basics, they aren't a kata. I don't consider the fuyugata and taikyoku type kata to really be important as fighting kata, they're just groups of basics strung together. Obviously, if you don't know your basics, no kata is going to mean anything. The one kata I'd want to have, if I could only have one, would be the one that encapsuled the strategies and combinations which had the greatest depth/variety. for matsubayashi ryu, this would be either kusanku, gojushiho, chinto, passai, or maybe naihanchi. Each of these kata could be their own seperate "styles", and probably were at one point. You can extract basics from each of them, and drill them with partners and on bags/makiwara. The basic kata of various systems, including the pinan, are exactly this...techniques extracted from the traditional kata and reformed into shorter/simplified drills.
It's hard for me to choose one kata, but I think I'd pick Chinto. I like the shoulder/elbow bump, the arm dropping, the spinning techniques, and the simultaneous block/strike...reminiscent of some other arts I've studied.
If I could have two, I'd throw rohai in there, too (the matsubayashi/tomari version). I just like it, even though it's really short.