There is no 'best', you'll have to decide yourself what to use in the end, all people can do is give their opinions on what they feel works best. It depends what you want from your brake, personally I find magura mineral oil works fine, and prefer it as it's designed specifically for the brake. If you feel the lever is returning slowly, then you might want to consider bleeding it with water, or a thinner oil, such as WD40. The best thing to do is try and ride other peoples bikes, and see how their brakes feel, but renember the rest of the setup (pads, booster etc.) also affect the feel. Is there anything wrong with your brake? Or do you just want to try something new or what? If it's not broken, don't fix it! Alun