For deep cleaning, I like Leatherique. I just did their process on my Q45 and was surprised how much dirt came off the leather. Nasty.
Leatherique is best done annually, maybe a little more often if your car gets really dirty. For normal maintenance I love the Zaino leather care products. Not oily or shiny, leaves the leather looking good and smells great. Cleaner is mild but effective. An ideal frequent care system.