First of all, if your leather is tan or gray, you'll wanna be extremely picky about what you use. Some leather cleaners can discolor the leather. If you have black, you won't have this problem. If you really want a good product, go to a feed/farm store to the saddle department. Just ask someone there for the best product. They'll know more than any auto parts store about leather care.
I use whatever I can get my hands on. I haven't found anything that works vastly superior to anything else. Even the Armor-all Leather wipes work well, but I've never had any deep-down grime to deal with. Just expect to do 2 or 3 applications for the tough spots. Each application will soften it up deeper each time.