My favorite (for several years now) has been the Swiss Triplet. Worked on making it smooth right to left on a practice pad for almost a year, just during warm-up stuff, before taking it to the kit. I'm a lefty, play open handed & alternate L & R set-ups. When I incorporated the Swiss Triplet into my playing, I finally figured out how to play the Purdie Shuffle Correctly, and the Jeff Porcaro groves correctly & some of Bonzo's Grooves correctly! It also opened up playing Flamaques, Flamaccents & Flamadiddles, forward and backward around the the kit without effort. It opened up a linear ability that even an ambidextrous guy like myself had never known! Suddenly, it didn't matter which direction I was going on the kit...everything was equal...well sorta LOL! Also started matching & reversing (left lead hands - right lead feet) the same rudiment with my feet about 6 months in to the practice pad work. This, as well as the paradiddle, is an amazing rudiment to pump your playing. Check out Dave Weckle's "Back to Basic's" series, that's where I got it and he shows the proper technique to play it. I really don't like Dave, he is a full blown "Richard" but he plays his a$$ off & is not a half bad instructor.
I consider the flam the most useful rudiment though...wether you're tapping, accenting, diddling or queing...throw a flam somewhere in there with it & you've just added the ability to play another drum at the same time without losing/using anymore energy. When you do the same thing with your feet, wether integrating them into the rudiment or alternating hands to feet, it's some cool $hit!
OK, you all owe me $20 for this lesson! PayPal only & send as a gift. ;)