I'd say it really just depends on what sort of things you watch/enjoy outside of anime. If you like action, then watch something action-related like Black Lagoon. If you like sci-fi, watch something like Akira.
If you don't have any interest in mind and just want us to speak from experience, I started out watching Ghibli movies as a kid, and I'd say those films are very "beginner-friendly." Spirited Away is by far their best work, and I'd also recommend Castle in the Sky and Kiki's Delivery Service.