Ordinary Differential Equations
(Introduction)
Any calculus text should offer a basic introduction on ODE's, but this isn't really necessary.
(Undergraduate)
Ross' differential equations is the best book you could wish for. Be sure not to buy his "Introduction to Ordinary Differential equations" which is shorter but doesn't contain the exciting material later on.
Simmons is good too, but is heavily plagiarized.
(Graduate)
Arnold's Ordinary Differential Equations is a masterpiece. But not everybody enjoys his style.
Teshl Ordinary Differential Equations and dynamical systems is very good too. A draft version is available for free:
https://www.mat.univie.ac.at/~gerald/ftp/book-ode/ode.pdf
Partial Differential Equations
(Introductory)
Any ODE book such as Ross will give you a quick intro on PDE.
(Undergraduate)
You can go wrong with Bleecker and Csordas:
https://books.google.be/books/about...al_Equations.html?id=tVXXD8sJ7uwC&redir_esc=y
(Graduate)
Evans' Partial Differential Equations is amazingly good, so you might like it. I personally prefer "An introduction to Partial Differential Equations" by Renardy, Rogers:
http://www.springer.com/us/book/9780387004440
(Beyond)
Taylor's multi-volume work contains everything you ever want to know on PDE. It does things from the right perspective too: manifolds. I recommend a good knowledge of differential geometry before tackling this:
https://www.amazon.com/dp/0387946535/?tag=pfamazon01-20