I reckon the best's Jay-El (by Joseph Liddy) - aussie stuff. All stations (ranches in the US) use it for all purpose softening and preserving of everything leather - saddles, bridles, boots etc. - stuff you have to really rely on. It's beeswax based. I love it.
Another very good one's neatsfoot oil. But it MUST be pure, if it (or anything else for that matter) is blended with mineral oil (if it doesn't say pure, it's blended), it will rot stitching. But it does a damn fine job for the leather.
If I want to waterproof - Nikwax. You put it on with your fingers to help penetration (warmth).