The most difficult QFT textbook I've encountered so far is Zee's "nutshell". Maybe the nutshell was too small for the vast amount of material he wanted to squeeze in, but the bottom line is that it is so imprecise that it becomes difficult to understand. Weinberg's books are at a high level but very easy to understand, because everything is very carefully explained. It's of course not so good as a first encounter with the theory, for which I'd recommend
M. D. Schwartz, QFT and the Standard Model, Cambridge University Press (2014)
L. H. Ryder. Quantum Field Theory. Cambridge University Press, Cambridge, New York, Melbourne, 2 edition, 1996.
or, if you prefer a path-integral-only approach (but this done really brilliantly)
D. Bailin and A. Love. Introduction to Gauge Field Theory. Adam Hilger, Bristol and Boston, 1986.