I don't know about BEST, but I'll give you a candidate for WORST ... I have a 1 year old John Deere Z920R that has what I believe to be a warped deck ... it cuts with a small ridge at the overlap of each pass ... I bought the little tool for leveling the blades ... and I got them to be near perfect ... ON A HARD, LEVEL SURFACE. Once I start mowing ... the little stripes show up again ... SOOO ... I took it to the dealer (RITCHIE TRACTOR - ATHENS, TN.) ... and explained the problem to the SERVICE LADY ... waited one month, then started calling about it. After several calls, she said it was DONE !!! I asked her what they found about the warpage ... she acted like she never heard me tell her about that, BUT ... they did level the deck, and wanted $84.00 for that service !!! After much ranting & raving, she finally gave me back my mower ... took it home ... fired it up ... SAME PROBLEM !!! They never addressed the real problem, but wanted $84.00 for something that wasn't even wrong !!! The Z920R may not be a lemon, but my dealer certainly is ...