There is no default ability for most spells.
Each class sets it's own spell casting ability. Thus for a Warlock, the proper ability is Charisma.
If you multiclass and gain spells from both classes, you use the spell casting ability from the class from which you gained the spell.
The spell casting abilities for each class are listed in the description for the class, in the Spell Casting ability section. Note that in chapter 10 it regularly refers to your "spell casting ability" and the following text is repeated throughout the character caster descriptions:
Use your [ability] whenever a spell refers to your spellcasting ability. In addition you use your [ability] modifier when setting the saving throw DC for a [class] spell you cast and when making an attack roll with one. (extracted from Paladin, on page 85 of the PHB).