I too have been on a quest to find a good leather cleaner, and agree that it seems ther are endless options. Have not found a favorite yet, have some more on the way after the recent AG sale, but here are my thoughts on what I have used. For all products I used a soft bristle raceglaze brush and a swissvax brush, wiped off with MF towels initially.
CG leather cleaner. Diluted per the bottle. Was easy to use, foamed up a bit. Wiped off fairly clean, but left the surface tacky. Required a couple follow up wipe downs with a damp terry cloth towel and then buff dry. Too much effort. Abandoned this one.
Duragloss leather cleaner. Nice product, but also left slightly tacky surface that required going over with damp terry towel, but only once. Came off cleaner than CG.
OPC. Diluted about 7:1. Dampened seats with a mist of water first, then sprayed OPC in small area and agitated. Very strong cleaner, even at that dilution. Still went over it with damp towel afterwards. This is process I would use on seats that have not been cleaned in a long time.
Ultima interior gel. My favorite in that after a MF wipe, seats felt smooth and clean. No residue. One odd thing in my experience with this product. While agitating, the product dries out very quickly. I now agitate in about a 6" square area at most, and saturate the brush before agitating. I normally spray the brush with all products and immediately wipe the area that I misted from the cleaner to avoid runs of product on the leather. Great results from this product, but a little more picky to use, great scent. Might be the warm dry climate here in SoCal. Wondering if others have experienced this.
Seats are black BMW Dakota leather. Next up will be Sonus that is on the way, then maybe leathermasters, then maybe leather Doctor. Until of course I find the one that vanquishes all others and I can quit buying products that get only a couple uses.
Leather Protestants/coatings, now that is another ongoing quest.
Oh, also, leatherique. Totally involved and time consuming process. Amazing results on my last car as it got older and seats experienced more wear. Current car is much newer, and leatherique didn't show the same kind of result. Excellent smell though. There are lots of opinions and threads about the usefulness of something like leatherique on newer coated leather seats, which topic is a bear that I don't want to poke with a stick here. But my experience with it on this car wasn't worth the effort required at present.