The Ranger Titan is the only version with a ranged attack, and their shots hit an area roughly the same as a Fireball spell. Their skills are a mixture of offensive and defensive boosts to their troops, typically of the same race.
Bar none, this Titan class is the best at general combat in both attack and defense, mainly because it is utterly unequaled at placing its damage output wherever it wants. Outranged only by Grenadiers (and by a rather tiny amount at that), in the time a Berserker or Infiltrator Titan can run a Ranger's weapon distance they'll have been shot twice, and dropped. For any of the other Titans it's three times, and also likely dropped unless they are heavily toughened. One of these backed up with a pure Grenadier army, as long as they don't spawn completely surrounded at point-blank range, can handle equal-level armies far larger than their own. Keep the Grenadiers on holding their position and shooting everything that gets close while the Ranger stays nearby and focuses on key tactical targets.
A high-level Ranger Titan as a defender of a zone is the only Titan type you are pretty much guaranteed to have to use magic against; if you don't and they manage to kill your own Titan first they will put your army through a world of hurt.