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Weight would be number 1 in order of importance - people keep saying "this is the year when the winner will carry more than 11st" but they've been saying that for the last 7 years at least. It might well happen this year but that doesn't mean they're not at a huge disadvantage.

2nd would be age - rule out all runners younger than 8 and older than 12 with confidence.

Other criteria are of less importance and you can be flexible - don't totally dismiss a horse if it fails on one stat for example (apart from weight or age), but ideally you want a horse who was proven stamina (has won over 3m 1f or further) and can jump in big fields (placed in a chase with 14 or more runners).