If you only want to buy one for starters, and you have some friends to play co-op with, I definitely recommend DR3. It's insanely fun, with tons of crazy combo weapons to discover and really doesn't get old.
Ryse is great (imo), but if you're only getting one game, I don't recommend it, because while it's tons of fun for say 3 hours at a time, you will need a change of pace from it now and again before going back to it.
ACIV, if you're an AC fan, and you enjoyed Naval in AC3, is a solid choice. It's not the prettiest game, or the most insanely fun....but it's solid good fun, has lots of content (as you said), and therefore would keep you busy for a while. It's a great pirate game really. Just not a next Gen game to write home about.
I'm not a fan of Zombies much either, but DR3 is a total blast. I'd say that's your best pick







