good question, most ZXR's i have seen have very random selections, if anything mate, go the softest rubber you can - these 45hp 250's have such a linear power curve you will never 'shred' soft tryes like a much larger bike will.... however if you have the zxr as a commuter, go a rear tyre with a hard center and soft outer rubber - will save you fuel and give you grip for the more spirited side of riding when its needed
Forgot to add the factory tyre sizes: Front tyre size: 110/70 17R Rear tyre size: 140/60 18R (Using a 140/70 18 opens up more options without throwing the speedo out by much) The rear factory tyre size seems to be very uncommon. I've only seen one Jap tyre maker carry this size - Bridgestone BT-92. Shinko (Korean) have tyres in all 3 sizes mentioned but aren't popular here in AU and for the price you pay, you may as well get something else. These three models keep popping up and seems to be the best all-rounders: Pirelli Sport Demon - Reports of being 'grippier' Bridgestone BT45 - Dual compound and oldest formula(intoduced in 1999?) Avon Roadrider I agree Cal, it's probably much of a muchness when it comes to which tyre to get considering the power delivery at the rear.
Have found that the Avon gave less feedback to the bars on a ZX600 than the 2nd hand Pirelli it was replaced with. Have Bridgestones on my 500 and consider them ok, but would still like to try the Pirellis to compare the 'feel'. Have Dunlop Roadsmart 2 on my ST1100 and they are ok for a heavy bike. It is all about how you consider the feedback from the tyre to suit you/cost/wear, etc.