Roughest? Definitely Ash. Even though May never even wanted to be a trainer, and one may consider that a rough start, IMO she still had it easier than Ash. Seriously, oversleeping thanks to broken clock, being ridiculed by your rival in front of his (and probably your) friends, scolded by Professor Oak, getting Pokémon, who doesn't like you, standing in pyjamas in front of most of your mother's friends, being hit by pack of enraged Spearow and being joined by bitchy girl, who only joined you because your Pokémon scorched her bike in attempt to save you? What ever can beat this as worst start of Pokémon Journey?
Easiest? I am tempted to say Trip, but he most likely will not count as main character. In all seriousness, though, Dawn. She knew all rookie needs to know about training Pokémon, she already had her goal and she was joined soon by two friendly and experienced trainers. Yes, she had it quite easy.