I would like to create a ranged damage dealer. Which class should I pick?

So, I heard you like doing damage at a distance?

Currently, the three best classes for ranged damage using weapons are rangers, rogues and artificers. There are advantages to each and you should pick the one that better suits your play style. However, you have to buy the artificer from the DDO Store or earn it from House Cannith Favor if you don't have it already.

Artificer
The artificer is a versatile class that combines ranged weapons and a companion with spells to create a potent damage dealer. The artificer can also disable traps. The artificers weapon of choice is the crossbow. There are several types of crossbows, but the most favored types are heavy repeaters and light repeaters. If you can find a heavy and light repeating crossbow that are the same, the best choice is the heavy as it does more damage. The artificer prestige class grants an ability to fire any crossbow (including repeaters and great crossbows) endlessly for six seconds several times per rest.

Artificers are additionally spell casters. The artificers have repair spells, so you can heal yourself if you are warforged or take the feat Construct Essence which makes your character part construct while allowing partial healing from regular healing spells. You also use these spells to repair your pet, who is a construct. Artificers spells are primarily weapon and armor buff spells, such as Enchant Weapons, which can make you do more damage or increase your and your party member's defensive abilities. Artificers also access some of the greatest offensive spells, such as Blade Barrier which can do alot of damage.

Specific to ranged combat, Artificer is able to add ("imbue") damage to the bow from Runearms--which may also add ranged damage as well--and apply their Intelligence bonus either to hit (Insightful Strikes) or to damage (Insightful Damage). Other spells may add elemental damage of your choice (Elemental Weapons), double your bow's base damage (Deadly Weapons), or modify the damage you or your party does to break damage reduction based on metal type or alignment. Other spells contribute to ranged damage including spells to stun or knock down enemies (Prismatic Strike or Tactical Detonation). Additionally, the battle engineer prestige enhancement adds +2 to the enhancement of all weapons adding to both damage and to-hit.

Artificers--like the ranger's arcane archer prestige enhancement--may conjure ammunition magically to simplify logistics.

Ranger
The next class is ranger. After all the artificer can do, you probably are wondering how the ranger can hold up. The Ranger can hold it's own. Rangers combine healing, self buffing and bow damage to destroy the enemies. But, in addition to that, rangers can dual wield weapons and do some awesome damage. Rangers gain evasion which is a very useful feat that can reduce incoming damage in certain ways.

All rangers gain the bow strength feat at level 1 allowing them to add their Strength modifier to damage from bows. Artificers and rogues may add their Intelligence modifiers to damage, but in order to do so artificers have to cast a spell and rogues have to select the rogue mechanic prestige enhancement and use crossbows. Like the other classes, rangers may also add damage or break damage reduction by using special ammunition types that are purchased, crafted, or imbued.

There are two options for ranger bow damage. The tempest is the melee line but can still do some respectable ranged damage. Since this is about ranged damage, we will talk more about the arcane archer, which has the possibility to have the highest damage in game for 30 seconds every two minutes. This is called manyshot. In addition to that,rangers can use imbues to boost damage. The best one is a slayer imbue, which on a vorpal hit (a 20) can instantly kill or do 100 damage. Combined with manyshot, you will see some BIG numbers. Rangers also gain 5 favored enemies, which (without enhancements) does +10 damage to 5 enemies which is a very big increase.

Rangers can also cast spells. Well it's not as robust as the artificer spell book, it has some very useful spells. The ranger has healing spells that are more than enough to keep you up. The ranger also has some useful buff spells that can help you and your party.

Rogue
Although ranger and artificer are more common, the Rogue Mechanic can still pull out some great damage, and so I believe it is worth mentioning. The Mechanic's main weapon is the crossbow, just like the Artificer and the Rogue can disable traps as well. Rogue mechanics add their Intelligence modifier to crossbow damage like Artificers casting Insightful Damage. Additionally, the Rogue does something called sneak attack damage. And not just a little, but 10d6 of it or more for halflings at level 20. If you are not familiar with D&D dice, that is 10-60 damage. That adds up quickly.

The bad thing about sneak attack damage is it only applies if the monster isn't attacking you or isn't aware of you, so trying to avoid getting them to attack you is often a full time job. With a high-damage producing weapon like a repeating crossbow, this is difficult unless you employ equipment that moderates aggravation like deception, improved deception, blinding, or Nightshade Venom (to induce sleep). Rogues also get evasion like rangers, and can get improved evasion which mitigates a lot of trap and many spells' damage.

Now, although rogues by default don't get to cast spells, they have the ability to use something called UMD to cast spells with scrolls and wands. This essentially allows rogues to cast any spell in the game with a high enough UMD. But, at high levels you cannot buy the scrolls from vendors, so if you want to use a high level spell you have to buy the scroll on the auction house or find it in chests.

Pure rogues cannot magically create ammunition like either artificers or arcane archers and must purchase ammunition. For rogue mechanics, this can get very expensive and take up a lot of inventory space as some quests will require 1000 or more bolts to complete. For ranged rogue mechanics (and non-arcane archer rangers), attaining the second tier of House Deneith favor to gain access to large wide quiver and sturdy ammunition (75% returning) simplifies the logistics for playing a rogue mechanic.

Conclusion
So, as you can see, all choices are respectable in their own right. All three can do awesome damage and have the potential to self heal. But, the most important thing to remember is that this is your character, and you should make sure to make what you enjoy the most.