Each has its own advantages and weaknesses, so a general answer is impossible.

Slaves - great for robbing (without war machines), basically carts for mercenary points

Cataphracts - biggest HP of all (well, leave out the trebuchets here) units in the game, heavily armored, it will work great in close combat against the enemy's meat shield

Teutonic knights - great cavalry with high HP and attack, good in close combat as long as it doesn't face pikemen

Swordbrothers - solid unit with high attack (for a not-mounted unit)

Templars - secret (or maybe not so secret) weapon against the enemy's cavalry!

Hospitaller Knights - quick and good against barbarian meat shield

Barbarians - meat shield unit for those players who can afford it

(Just to give you an impression.)

Most bigger players use (almost) all of these units. If you don't have enough mercenary points to do that, look at the mouse-over texts and decide which units go well with your style of fighting.
I also strongly suggest that you have a lord or any experienced player who helps you with these things.

Quote dpopovic:

Those are mercenary from other servers.In El Dorado they are different.

My bad, I'm sorry!

I don't play on el dorado, so I can't help you when it comes to the units that are only available on this server.

Quote schnitzer90:

look at the mouse-over texts and decide which units go well with your style of fighting.
I also strongly suggest that you have a lord or any experienced player who helps you with these things.

This is still true though.

How can you increase the population to reach the last stage the obelisk calling 2000 even kill all the soldiers in the city reaches 1900???
To release and mercenaries of the obelisk.

I vassal 2 free points Vassal but I do not know how to recruit them?

I have too much buildings, probably this is the problem, the farms is all maximum ! Thank you !

And how occupied the Mercenary camps ?