My path was an early path of fits and starts:
I read Smooth and Striated out of ATP before I knew anything about anything.
I revisited that a year or two later, alongside the Introduction to ATP; and again a year or two later still. At some point, it started to really make sense and fit into my brain.
After that, I jumped into Cinema 1 and 2, which feels incredibly relevant for thinking about my topic, Google Street View
For my exam reading list, I also read (in addition to C1 and C2, again):
What is Philosophy
A-O and ATP (also with our reading group, Becoming Poor)
What felt like way too many secondary sources at the time:
Flaxman: Brain is the Screen;
Rodowick: Time Machine and After Images;
I then read Bergsonism and Hume for a paper on Duration.
Then for fun, I read
Spinoza: Practical Philosophy (agreed, really helpful);
a large portion of Nietzsche;
Immanence: A Life essays;
From there, I developed a course proposal which focused on some of the key concepts of Deleuze, which forced me to dig a little deeper, read a little wider, and brought in a lot of his sources that he references for the concepts I was covering. That was extremely fruitful.
This past spring, I took a philosophy class on Spinoza, reading The Ethics. I think this has been one of the most helpful aspects of understanding Deleuze at a completely different level. I wrote a paper about Wonder, using Deleuze’s Expressionism in Philosohpy, and returning to Practical Philosophy, placing the role of wonder alongside the concept of deterritorialization. I’m screwing up the courage to try again to read Difference and Repetition, but the timing isn’t quite right. I have a sense that it will be far easier, after tangling with Spinoza.
My advice is to read broadly to get a sense of how he talks about things, then focus on something that feels relevant. I think Deleuze’s own advice is that we necessarily start in the middle, and finding something particularly relevant for one’s own research seems like the best way to start. It will resonate, for one, and there are a host of secondary sources/dictionaries that can elaborate on key concepts. I think the key is repeated exposure. There is only so much we can glean from a first read.