Best case: maximum value somewhere in the 7's (or less, but sometimes that is indicative of basal problems for me, so I get a bit nervous if I eat and don't spike at all. Obviously it's a good result, but usually it means I'll be headed downwards soon after)
Acceptable: depends on the time of day. If I know I will be doing some activity e.g. getting dressed, showering then I will be ok with a higher number because I know I will drop. Recently I've been getting 8's-10's after breakfast, which I wasn't happy with, but they tended to drop back down quickly after just a couple minutes of incidental exercise. If I'm not exercising, then I will be ok (although slightly disappointed) with anything between 8 and 9. Over 9 is too high, in my case it is almost always due to basal so I will adjust that, or else I will exercise around that time to help stabilise a spike.