I used to be a huge robot nut back when I was really young, but I don't draw them as much now since I moved into doing creature design. However I recently got back into doing robots again when I decided to do an Overwatch fan comic in order to push my work up to a higher standard. I eventually completed a design for a tiny robot that doubles as a music box.
The kind of robots I like are more steampunk/clockpunk contraptions, elegant little devices often with a lot of needless decorative bells and whistles, (think the clanks from "Girl Genius" or the Seconds from "Alice - Through the Looking Glass"). I think it's nice to be able to draw robots that have a more organic feel to their shape language. Maybe that's just because I'm not great at drawing blocky geometric forms due to my more animated style. I'm also quite fond of the Machinarium robots, and some of the designs from Blue Sky's "Robots" film. As for Overwatch, I feel many of the omnic designs are varied and unique, although some I have mixed feelings on.
What I did learn about designing robots from the project I'm doing is how important shape language and silhouette are compared to adding "greebles" (details that make it look like a functioning machine). I recommend learning a lot about silhouette and shape before you consider studying robot "anatomy". The complex mechanical parts will be easier to pin down if you can get the other fundamentals out of the way. It's also worth exploring industrial design and real-world robotics to at least learn a thing or two about what kind of greebles to put where, not that your robot actually needs to work in reality, (I mean let's be honest, all robot character designs with a lot of detail are made for the illusion of functionality rather than actual functionality )