Well........ my personal favourite is Palm Springs, California.
I asked the question Gammy as it is always interesting to make comparisons. Alas, I've no experience of Palm Springs.
The truth of the matter is that in my experience virtually all airports are a bit grotty. It is all down to the problem of letting Joe Public use them. Prestwick is by no means perfect though it is better than it was. It does not, however, deserve to be singled out and certainly does not warrant the word 'disgrace'.
I use both PIK and GLA a fair bit and had fewer 'bad' experiences at PIK. One example last year at a temporary cafe at GLA was particularly bad but I wouldn't completely slag off Glasgow on the back of it as you seem to have done based on one bad Prestwick experience.