In 223 Remington with a 1 in 9 twist rifling what is best bullet weight?

1-9 is a very versatile twist. It should be able to handle the varmint bullets above 40 grains, and keep you entirely stabilized up to 62 grains.

It's not so much the weight of the bullet which matters, but the length. I would think that anything under 77 grain OTM might be just fine, but...Over 62 grain I would be careful.