The only reason Season Seven isn't number one (which pains as an big Ezri fan) is that I feel there is a slight dip in quality in the middle, unlike Season Six.
Six is the one with the best overall consistency, IMO. There's only two episodes in it that I don't think are at least above average - Valiant
and Profit and Lace
- and they're back to back.
So, if Six is a 10/10, then Seven is a 9.5/10.