For overall content it has got to be Duchess. Just an incredible hard carry champ that scales throughout the game and has a place pretty much everywhere.
No other champ changed my game like Duchess, and I still use her on most dungeon and dt teams. I use Ma'Shalled more and more as well. I like to pair BEK with Rae. He cleanses my team and she strips my enemy. The two of them with Duchess gets me through a lot of content with a few spots to flex.
There are too many great ones to say who'a the best. Ain't a horse can't be rode, ain't a cowboy can't be throwed.
It may be the shiny new toy bias, but Ragash is right now my favorite. He protects, he debuffs, he CCs, he nukes, his stats are solid across the board. And he pairs well with the rest of my team.
Another thing influencing the bias is that I have a lot of good support/protect champs, so the context is different. If I didn't have Krisk, Tuhanarak, Helicath etc. I bet Duchess would be my savior, so it's also up to whom you use the most based on your account needs.