Lexol is probably the most popular, that's what I use. I think Meguires also has a leather cleaner conditioner all in one. Lexol is a 2 step process, while the Meguires is one. I haven't used the Meguires but most of their other upper end products seem to be pretty good. I use Meguires vinyl cleaner for the other parts of the interior and it's been the best product I've found so far.
DON"T use armor all or formula 2001. Anything with silicon in it will harden your leather over time. I'm speaking from experience. Bad exerpence on my old G1. Lexol is your best option to clean the leather. Follow it up with some eagle 1 pre-cleaner then with the eagle 1 leather conditioner.