There are frame sliders, engine case sliders, and axle sliders. The best brand is somewhat subjective - they are not typically tested head to head. I chose OES because they were were proven to work, and affordable for Frame and axle protection. OES is basic but solid design. Other companies like wood craft make excellent bits too including engine case sliders with replaceable pucks. Many have gone with brands such as T-rex, shogun, R&G, evotech - all are pretty easy to install.
remember without engine case covers, you will get rash or damage on them in most drops.
+1 on OES. The full kit: frame sliders, front axle sliders, and rear swingarm spools cost about $130 I think. Great choice, and some people on here have put down their bike with them will no damage except for replacing their rear brake lever.
Protecting my 09 is worth the extra money in my eyes.
p.s. - not sure I like the idea of the engine case covers that completely cover the cases? I don't like the idea of trapping more heat in, like insulating the engine on a hot day. I like the one that still let the engine cases breath.