I classify my land units based on their primary combat roles, and I've come up with three basic classifications (four if I count "Garrison" as a combat role).
Ranged/siege units are best at reducing city defenses and destroying enemy units without taking damage.
I use my melee units to screen my ranged units from enemy units. Melee units have a secondary role of capturing cities once ranged units have battered defenses down.
The movement points of mounted units makes them ideal for a role I like to call "active defense", that is, actively engaging enemy units that are freely roaming my territory in groups of two or less.
I've pictured Civ V units to be equivalents of US Army battalion/cavalry squadron sized units.
Thus, the organizations I use to seize cities are task forces that are roughly brigade sized.
I tend to use two organization schemes: one for the Mongolians, and one for everyone else.
For everyone except for the Mongolians, a typical task force TOE is:
1+ melee unit to soak up ranged fire and to prevent enemy melee units from destroying my ranged units and to seize the city once the defenses are down.
2+ ranged (pre Renaissance/no iron)/siege (with iron/post Renaissance) units to batter down city defenses.
1+ melee unit in reserve/healing
1 Great General
When I start reinforcing the base formation of five units (2 ranged, 2 melee, 1 Great General), I tend to use a ratio of 2 ranged units for each melee meat shield. I've found that my ranged/siege units are too exposed to counter assaults by enemy units if I have more than 2 ranged unit per melee meat shield.
Finally, I tend to attach additional Great Generals for every 5-7 non Great General units to ensure that every single unit in the task force has the Great General combat bonus.
Obviously, the Mongolians require a modified task force organization, given the unique abilities of the Keshik.
3+ Keshiks to batter down city defenses
1+ Mounted Melee unit (Horseman/CS gifted Knight/Cavalry) to grab the city once defenses are down.
1 Khan
Optional spotter melee unit
The skirmishing siege ability of the Keshik means that melee meat shields aren't required to prevent counterassaults, although spotters may be necessary, dependent on the terrain.
My pre armored mounted units are only attached to my task forces on an as needed basis, because I've found that tying them to forces task organized for city sieges needlessly wastes the one strength of mounted units: their mobility.
Naval combat is somewhat irregular for me, so I don't really use any organization for my ships. The same goes for my aerial units. Once upon a time, I tried to keep to an organization of three aerial units (squadrons) maximum per city to mirror the US Air Force 3 squadron/wing scheme, but I found it to be cumbersome to maintain.