I'm ordering the champs by their base level range.
(650) Alright, well Caitlyn is definitely one especially with her passive a great auto attack harasser.
(600) Ashe, with her passive getting that first 100% crit on the enemy and with a volley is almost 3/4 their health.
(550 + 9/level) Depenging on the level, Tristana is a good one also since her passive gives her more and more range the higher the level so she may not be the best harasser early game. She starts with 550 range and ends with 703.
(550) Miss Fortune not only has decent range but she also has a very fast animation which makes for great harassing you can get an attack off before the opposing.
(550) Corki is another with a great passive for harassing since he will deal an addition 10% damage.
I know you mentioned you don't want to use spells but Kog'Maw has to be mentioned.
(500) Kog with the use of his 'W' (Bio Arcane Barage) he increases his range by 130 / 160 / 190 / 220 / 250 , depending on his level and does additional damage base on the targets max health.