Hard to say what my favorite is... But here are some of my favorites that I continue to return to and why I love them: (in no-particular order other then my personal 'world map order'
Canada - a very simple Civilization that dosen't really have anything that stands out. However I absoltely love making the best empire possible with as much tundra and snow as possible. I am a builder/city-planner type of player, so to be able to take barren tundra and turn it green and lush and productive; that's why Canada is one of if not my favorite.
Maya - I love huge expansive empires where the sun may never set (I am British after all) but the Maya gives me reason to focus on a small handful of 8-9 cities to make then pretty and tall. It's a nice Civ for me to play as when I want to kick back and just see how other players and AI get on in the game. Also I enjoy figuring out how best to build a network of farms and get the most adjacenies for my Observatories and other districts in such a confined space.
Gran Colombia - The +1 movement is great and unique, meaning your settlers, workers and early game units get much more done much quickly and the free promotion action is great for all militery units, spies, apostles and rockbands to also get to work immediately instead of having to waste a turn. But the best bonus is the Hacienda; I love make huge spawling hacienda-plantation compounds. The extra production, and gold is great, but I love the housing most; I love getting huge ammounts of housing and populations in my cities.
Inca - I love the Terrace Farms and their adjacencies to and for mountains giving you lots of food, production and housing from otherwise unlikely places and make the landsape to weird and unsual compared to other Civs. Because I don't want to sacrifice any terrace-farmable tile I enjoy the extra city planning to see where the best places for districts should be when keeping the number of Terrace Farms as high as possible.
England (Victoria) - Unfortunatly there is nothing intresting about the city planning as England; but I still enjoy trying to get at least one city on each continent either peaceful colonialisim or through agressive-diplomacy; and trying to make a empire where the sun never sets with super long railway networks and a formidable navy backed by cities fully powered with their own industrial zones. (Although it's so very difficult to make best use of Victoria in the most recent update due to the end of the world coming too soon; both with and without Apocolypse Mode enabled.)
Netherlands - Polders. Need I say more? Ok, I'll say more; the city planning for your other districts along rivers is also really cool; and provided you have enough viable rivers, you can get a solid +2 or +3 adjacency for your science, culture and production districts without needing to worry about their regular adjacency bonuses.
Spain - I love city planning; and the best part of a civ for me is normally the tile improvements or the districts that give cool and intresting adjacencies. The Mission is no acception for me as I use to love to spam it around all campuses for so much faith and science (and they all look so pretty!) but with the most recent version of the mission they now need to be next to both campus and holysite for the best output; but with the bonus poduction and food from being other continents; even more reason to unlock my inner Victoria and colonise a whole other continent~
Hungary - I love the culture, leader and music of Hungary, so this Civ was always going to be a win for me; but I love cool city-planning aspect of the Pearl of the Danube ability which means you can shave off precious turns of production in your cities to build these nice little urban settlements along rivers which look so cool.
Mali - I love trade routes. Probably just behind food and housing for me, trade routes are my third most important thing I focus on in my games; so the fact Mali is based around trade routes and buying everything with gold or faith really fits perfectly in my play style as I do this with any Civ I play as; so Mali is just made for me.
Persia - Bonus to trade routes, a unique tile improvement that gets cool and intresting adajencies from districts and provides appeal meaning very cool and intresting city planning. There are some war bonuses too which I don't really care for but can help when they are required, but otherwise I very cool civ to play as for me.
Khmer - I LOVE the Khmer. I love the fact it's the only Civ (with acception to Rome) that REALLY gives you reason to build aquaducts. This Civ is also perfect for me, as the two things I focus on mostly with my games is Food and Housing; and the Khmer has that with bonuses to faith and amenities too meaning I can challenege myself to get the tallest cities possible. (no matter if there are pops that are unable to work anything such as those in my record 58 population city which still had overlap with other cities.)
Indonesia - I loved Indonesia in Civ5, so when I found out they were going to be in Civ6 I couldn't wait to see how they transitioned. They are basically unrecognisable but all for the better. The Kampung is hands down my most favorite tile improvement in all of Civ6 because it's so beautiful and you can spam it almost anywhere on costal tiles providing housing, food and production to otherwise un-improvable tiles. Yes, since Fisharies were introduced in the DLC just after Indonesia's inclusion, these are still better as they sacrifice 1 food for double the production and quaduple the housing and you don't need Liang. But I've always loved the appeal of stilt villages IRL, so they are always going to be my favorite tile improvements. (the rest of the Civ's bonuses are ok, but never really really notice them.)
Australia - Again, another intresting Civ to play as as you need to focus on city planning with appeal in mind adding an extra level to the puzzel which I enjoy. I also love the music but the best bit of all is the Outback-Stations. If I've not already made myself clear; I love Civs almost entirely based on their tile improvements or what adjacenies from districts, appeal or other mechanics do to the tiles around them, and the Outback station is another beautiful example of a tile improvement which can be spammed anywhere around pastures and in desert to make the land look so very unique and beautiful in a way that no other civ can.
