Commander pairings

A pairing is two commanders in one march. The first one — the primary — gives both skills and talents: his talent branches work for the whole army. The second one only adds his skills. That is why the primary slot goes to the commander with the stronger talent tree, the one you level first.
Below the pairs are sorted into tiers. Each pair says what it is meant for, and for the primary commander you also get the talent build screenshot.
The ones marked "for whales" really are strong, but putting them together without spending is next to impossible.
SS
5
Open field, medium mobility (any troop type)
Long-standing and strong until the next nerf — one of the best pairs in the game, all the way into the Season of Conquest. Level Æthelflæd's skills 5-1-1-1.
Talents: Æthelflæd
Barbarian forts
Æthelflæd at level 60; Cao Cao only needs level 30 — all of his skills are unlocked by then.
Talents: Æthelflæd
Killing enemy gatherers, fast flag captures
A very narrow, specialised pair.
Talents: Belisarius
build from Cao Cao — same talent branches
Top-tier cavalry (for whales)
If you have decided to sell your flat after all, this is the pair to play.
Talents: Minamoto no Yoshitsune
Top-tier infantry (for whales)
If you have decided to sell your flat after all, this is the pair to play.
Talents: Pyrrhus
S+
4
Open field, high mobility (cavalry)
If you play cavalry you can swap their places: the damage difference is minimal, but with Baibars in the primary slot you get a great deal of mobility.
Talents: Æthelflæd
Open field, medium mobility (archers)
A decent option if you have decided to skip Sun Tzu.
Talents: Æthelflæd
The pair you need for Sunset Canyon
Sunset Canyon barely works without this pair — take it into the canyon.
A very strong archer pair
But hard to level as a free-to-play player.
Talents: Thutmose III
S
2
Barbarian forts
Boudicca at level 60, Cao Cao at 30.
Talents: Boudica
Open field (infantry)
Bjorn in the primary slot for talents; Sun Tzu brings the AoE.