With your mention of wanting to spread faith, ARABIA is the best civ for you to choose by far in terms of faith-diplo civs. They even have one of the better attacking units in the game with the Camel Archer should you ever decide to play aggressive during the unit's lifespan. INDONESIA is pretty good at raking in faith if you manage to have enough Candis and the extra resources promotes trade, and therefore gold and diplomacy. Along the same lines of Indonesia, MOROCCO really promotes trade as well and the gold can add up although I'm not sure just yet if it's as good as what Arabia and Indonesia can get.
As for civs that really reward you for playing friendly with city-states, SIAM is amazing. You don't get any benefits straight up but the outcome is so rewarding that you will be motivated at all costs to have as many city-states on your side as possible. PORTUGAL as well does this as friendly city-states means more resources from the Feitoria (and even more if you're allied if I'm not mistaken). They also promote beelining to the Nau which means it promotes exploration, which is great for founding World Congress. Although along the same lines POLYNESIA is a sure-fire bet to be the founder of World Congress in the player's hands as you will have explored the map before anyone else, and since you mentioned taking advantage of the new UN, they will give you a step ahead and extra delegates. All you have to do is make sure your science is up and and beeline to Printing Press.
If you want to ally with city-states fast and make sure nobody takes your crown, GREECE combined with Patronage is pretty potent stuff. Never tried Greece on multiplayer though so I don't know if they are any good there. But even the most adept Greece player could have trouble against an equally adept VENICE player as their potential to rake in more gold than other civ means you can flat-out buy your city-states quicker than average. Really powerful civ for diplomacy and I know a lot of people's new diplomacy favorite. Then of course there is SWEDEN and with so many potential excess great people in the game you can gain CS favor fairly easily without spending a dime.
As you can see the options are deep and I probably didn't even mention all the potential civs. In the end it comes down to personal playstyle and preference. For example, Polynesia is a great civ for becoming World Congress founder, but a lot of people dislike them for their very different playstyle. But from what you mentioned I would try Arabia first. Plenty of gold to make which is crucial for allying with city-states beyond quests, plenty of faith to spread as long as you make sure you build the Shrine as early as possible. I'm not sure how new you are to civ, but if you are then I would avoid Indonesia and Sweden from the ones I mentioned and probably Venice as well.
EDIT: And I'm not a regular multiplayer player, so take my advice with a grain of salt and wait for someone around here who knows MP to give some more advice
