I don't know what you mean by 'middle of the sea'. The 'best' cruise line is purely subjective. What is good for me may not fit your tastes or expectations. I like Royal Caribbean and Celebrity because they are a good fit for me. Carnival offers a great product at a good price, and don't think that Carnival is a party line. They are as mainstream as Royal Caribbean or Norwegian Cruise Line, but the latter two lines spend a lot more money on innovative cruise ships.
I prefer the western Caribbean over the eastern runs. Again, this depends on what you want to do while ashore. The western stops have better snorkeling and diving. The eastern ports are more focused on shopping and typical touristy things in my view.