There's so many on the market, it's not so much a matter of which one is good, but more a matter of what style or material you want. They come in aluminum, carbon fiber, stainless steel, titanium, chrome, and so fourth. Some are simply slotted with open holes, or caged, while others have ducati motif designs. Price wise, they'll set you back anywhere from around $50 to well over $200. Although most of the metal ones are built well, some of the "cheaper" carbon fiber units are very thin and prone to cracking around the bolt locations. Some have aluminum or steel inserts within the bolt holes, and although they help, if the carbin fiber around it is still thin, they're useless. Off the top of my head, and brands I have purchased myself include; Cycle Cat, Speedymoto, Road Racing, MPL Tuning, and Ducati Performance products. All of which are top notch in quality and construction. The "Brand X" pieces you see on eBay starting at $1, I would be weary of those.
If you're on a budget, for about $25 most any machine shop can slot out your OEM cover for you. If you have a bench type drill press, you can even do it yourself, although a milling machine would be more ideal. You can then paint to match the cover, or have it powder coated. I know of many people who have done it, and the end result looks quite nice.
If and when you do decide on one, post some pics!