When choosing your army, you must nominate one model to be your Warlord. Unless specified otherwise, this must be a character model. If you do not have any character models in your army, then select any other model in your army to be the Warlord. The model you choose as your Warlord also determines your Primary Detachment.
So basically any character. This means that even a non independent character like a squad leader can be your warlord. Though as said above some chars say they have to be warlord. Somevsay they can never be warlord (i think a spacewolf guy has that). And in wierd situations you can have insane things be your warlord like the lead Helbrute in a Murderpack (as 1 of the 5 becomes a character and gains look out sir). Back in thr previous version of Eldar codex a Wrathlord had Monstrous Creature (character) as his type. And in Iyanden you can take a Wraithknight as leader if you wish even though it is not a character.