But seriously, difficulty is a relative concept. Are you talking just about intro QFT? Otherwise you could include books on N=4 SYM and curved space QFT which are obviously much more advanced than introductory topics.
For my first exposure to QFT I used Peskin. Now I'm on my second taking of QFT and I'm using Peskin as the basis supplemented with Greiner,Weinberg and Srednicki. You could start with Greiner too but I don't really like his style and only use it for a few things, such as the full proof of Noether's theorem omitted in Peskin. Srednicki is a good start too. All of these except Weinberg are roughly the same level anyway.
The book by Lahiri and Pal is probably the friendliest intro book I've come across though. It's very step by step with lots of problems.
Quantum Field Theory for Gifted Amateur is a very good book to start with. But you really need to read another book after that, maybe Quantum Field Theory and Standard Model by Schwartz. I think Hatfield's or Weinberg's come next. But most people here tell you that you really need to read Weinberg's so I guess you can forget about Hatfield's if you want or just use it as a supplement to Schwartz's. Anyway, I think Hatfield's is nice enough to keep it in the list.
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.
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