Warlocks, Shamans, and Witches.
It is among these spellcasters that you are most likely to find Eldritch spells being used. The primary difference between Eldritch and Arcane magic is in where the power comes from. Which in turn dictates how that power is applied to achieve a given effect.
Arcane magic works by subtlety adjusting the balance between Order and Chaos that underlies our reality, by directly interacting with those forces.
Eldritch magic in contrast adjusts that balance by introducing, (and in some cases removing again), Chaos from Elsewhere. Less subtle and elegant than Arcane magic, but when Chaos and/or Destruction are the goal Eldritch magic is easily as, if not more, powerful.
Now the reason that Eldritch magic is more likely with these spellcasters than others is a matter of how these casters gain their magic in the first place.
Warlocks (and technically Dracons, Naze, and the like) gain their spells by forming a Pact with a Patron of one type or another. And it depends on the Patron and the circumstances, as to which mode (Wizard, Sorcerer, or Cleric) will be used. Of course there are any number of Warlocks who will try to find ways of recording the spells they are given regardless of the mode the Patron selected.
Which brings us to Shamans and Witches. Unlike Warlocks, Shamans and Witches don't make an enduring pact with a single being but rather bargain with many different types of magical beings to gain the spells they need. This can lead to them becoming a Warlock, Druid, or Cleric, depending upon the type of spells they are most interested in. After all, why keep shopping around if you can get everything you're looking for in one place and for a single set of terms.
The most notable difference between Shamans and Witches is that Shamans are typically members of a Barbarian tribe and so tend to be illiterate. Thus leaving them little choice but to follow the Sorcerer path for learning spells.
Witches, by contrast, tend to live closer to civilization. On the edges of typically, but still, Close enough that reading is a skill that they can typically learn.