My three main men: Kassadin, Malzahar, and Karthus are all fantastic harassers.

But it's my main lady: Caitlyn that outshines em all.

Played right, and they'll never leave their turret.

How is Katarina a great harraser? She does her teleport move, you stun and beat her up a bit... thats not harass. A strong harass is like Ezreal or Tristana because of long range and blink abilities to dash around the battlefield without the need for an object.

Mordekaiser is the best

Maokai anyone?

i say kass is REA::Y good early, and WHAT? lol, temmo is good late game imo

AP Yi, recently trashed Cait, Brand, and Anivia, in mid. Amazing harass even if just because his meditate heals him 100%, so he takes 0 damage.

Kogmaw is free this week and his W and R have huge huge range.
Urgot is great as well but he's not free this week I don't think.

Im just going to say "Le Blanc" regardless if people disagree.

Q-W-E-W-R-Q-R (doubt you will get past Q-W-E-W if they are smart

W-Q-E-W

Im sorry to disappoint some people here but first of all: Everyone who has stated a champions combo, aka
W-Q-E-W or whatever.... Thats not harassment. Thats attacking. Harassment is when you use a single, generally ranged, skill that deals significant damage with little or no chance of a counter-attack. Harassment is not something that generally happens late game. It happens during the first few levels when neither player is able to go in and just kill the enemy so they try to slowly wheedle down the enemies health. I would also like to mention that if played right teemo is the most overpowered champion late game. I am so good at teemo personally that it has become almost boring to play as him. My personal opinion of a good harasser would be kassadin. Though it takes some skill to get used to a melee caster like him his Q is a great harass and late game he can take out tanks in a few seconds.

Urgot is a good champion with harrass with his combo. He can land his abilites from across the screen and even if he cannot see the target, as long as they are still under the effects of the gas.

Swain can harass pretty well against low mobility champions

Anivia has a high risk high reward play style (consumes a ton of mana, can do a lot of damage) dealing half their life in one harass combo if done correctly. The skill shot hits twice (once passing thourgh then on activation) then hiting with frostbite to get the double damage turns into at level 2 (60*2+55*2) 230 magic damage. Mind you this costs 130 mana.