Which civ is the best fully boomed civ for late team game with no rush? 4v4 team game. As in lots of farmers at least 40 to 60, 30 to 40 lumberjacks, and trade carts. Gold mines are gone, but they have at least 20 to 30 trade carts so they still have gold flowing in.
Granted, this may not happen all the time. Especially in pro game game where a rush happens. But what civ works good here when fully boomed and no rush happens?
I’d go for Mongols, if you can get to Mangudai and Siege (Siege Ram and Siege Onager) with all upgrades for both of them (which is A LOT of investment).
Other options would be strong elephant civs, mainly Khmer (their Heavy Scorpions are a nice addition to the elephants). Burmese are also underrated in that spot imo. Their elephants are very tanky and Arambai are very strong as well. Still think Khmer are a bit better ultra lategame, but possibly not by much.
Persians with War Elephants sound good as well, but probably are still too easily countered by Monks (even at this point in the game). Shouldn’t be a bad choice though.
Other civs with strong Siege are surely a good idea too (Celts, Ethiopians, Teutons, Koreans), Mongols should still be the best though, as their Siege as also amazing and Elite Mangudai are the perfect unit to combine it with (also adds a ton of mobility).
So I’d say:
1 Mongols
2 Khmer
3 Any of the other civs I mentioned
Indians come to mind:
There are definately some others you could argue are best.
Well as always: It depends.
On closed maps I would go for the no1 pick in the FN community: Celts. what can beat celtish onagers in addition with some halbs? Rigth! Bombard cannons, therefore we could add Turks into the team as well. Now to make trade more efficient Spanish would be cool, and last but not least: Mongols. Fast rams into enemy houses, so they are pop capped and cannot produce anymore (also cool: you are due to Nomads immune to these kind of raids)
On open maps: I have no clue 11. Maybe mongols, maybe magyars, anything with fast unit composition and hussars seems great for death by raiding
teutons, khmers, celts (oh read wrong, this civs best boom civs, not best late game)
well spanish means u have same amount of gold for less trade carts than ur opponent, and that applies to u and ur teammate, so that’s like ure team gets an extra 30pop space, that’s a big deal. And spanish have great late game options, fast firing Bombard canon, paladins, siege rams …
For fully booming and turtling? Portuguese reigns supreme. Since stone mines run out, and stone gets more and more expensive to buy… getting stone from Fetoria is a big deal. Since even one bombard tower already FORCES your enemy to spend gold on siege 
Rather than asking us, it is better you search for the answer. How? Simple. Look for power units - Paladins and equivalents. Elephants. Siege Onagers. Maybe even Bombard Towers. Atleast 50% civs should have Siege Rams too (SR without Siege Engineers is fine).
If you are looking for intersections, the most overlap is with Teutons which get everything except Elephants.
Another one - my favourite - Celts. Woad Raiders are just better Hussars. Especially against buildings. You might sneak in, destroy the TCs quickly and run away. Celt siege is surely the best. Celt Paladins are just like Leitis in terms of survival too, so you maybe pleasantly surprised.
In team games of 3vs3 and 4vs4, always remember to have:
While Cavalry Archers are good, you may still not make them for general combat. For raiding, even Berber Heavy Cavalry Archers suffice.
Indians: Best Castle age Boom in the game + Imp Camel to counter palas, the usual lategame uits in teamgames.
Teutons: Good Boom, Strong Units. Only Civ with FU Siege + Paladin. TK can be situational game winners.
Khmer: Good Boom, strong lategame comp with ballista eles + siege. Best dethball in the game
Koreans: Second best Dethball with war wagon + buffed siege. Advantage: Comp doesn’t need food leads to earlier transition, no lategame farms needed.
Saracens: Not as deadly as Khmer or Koreans, but Mameluke + Siege still one of the best comps i the game.
Italians: New GC are absolutely deadly vs cav. Deadly in comp with siege.
Celts/ethiopians: Insane Siege, but need to be paired with a good cav/camel civ to be effective. Maybe Cumans.
Cumans as they have in total the best boom can’t compete with the other because their lategame options arent the best. But they might be a good second pocket civ to have also some mid-game power whilst you try to boom with your lategame civ, especially with them who don’t have good cav options.
Spanish are good for trading but have almost nothing else going for them in the lategame
As you see, the most important point for a lategame team civ is siege + a good pariring with the siege to protect it from the enemy cavalry. Only Indians are the exception with their insane boom + imp camel (which is also somewhat siege, too).
Yeah, I think I was thinking too much about the very late game 
all those options are fine until even trees start running out from the map
Mongols are probably the most obvious with the best deathball with siege, UU and possibly hussar if the opponent spams skirm. Magyars, Frank’s, Britons, Italians, Spanish are all quality lategame mostly for different reasons but civs like Teutons, Celts, Mayans, Chinese and more have really good potential