Alpha10's (or GRP70's which is the tyre that pre-dates the Alpha10) are the traditional choice for the 250, along with Andy's favorite the Bridgestone BT090

But without doubt the best (treaded) tyres available(ish) in 250 sizes once they're warm are Pirelli Diablo Supercorsa's. They're HUGE money now over here (£350 a set full retail) but on track they'll give better feedback, be faster and out last 2 or 3 sets of BT090's or Dunlop Alpha10's.
You really do get what you pay for.
Pirelli Supercorsa Pro's are also available in 110 and 150 sizes...they're similarly good, slightly cheaper and a ton better in the wet, there's a set on my racebike (in 120 and 160 sizes) now that hold the Croft lap record*

But a 3.5" front and a 5" (or 5.25") rear will let you run slicks or inters or whatever you fancy.

*not set on my racebike or by me
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Last edited by fontyyy on Mon May 31, 2010 8:22 pm; edited 2 times in total