I'd recommend Vikings. Time for TV, however, is at a premium. I seem to only be able to watch in airports and hotel rooms.
I am surprised I do not see more people talking about Vikings. I think it is a great show. Of all of the non-premium cable shows (which would exclude Game of Thrones and Homeland) I think it is the best. Outstanding show and production from the History Channel.
That said, Game of Tbrones and Homeland would be my top two recommendations based on what the OP has already watched. Breaking Bad was possibly the best of all time. It had so much - comedy, high drama, action, great character development, solid acting and great writing. Simply wonderful. GOT and Homeland are my next two favorites of currently running shows, but Vikings is really solid as well.
I watch The Walking Dead and overall I enjoy it as well, I guess, but it is one of the most poorly written and directed shows on TV and some of the acting is horrible. Great idea for the show and the overall theme is unlike anything else, which I enjoy, but just poorly executed at times. The plot lines become repetitive as well. But, I still watch it because there is something about it that I can't look away from and move on. It doesn't hold a candle to GOtT, Homeland, or even Vikings, though. So I would watch those first.
Also, before I started the Walking Dead I would consider some of these older shows that are now completed series - Sopranos, Friday Night Lights, Mad Men, Lost, and True Detective Season 1 would all be vote getters from me. I enjoyed them all very much. All are very good series that I recommend highly.