Because it's written by a male and it is mostly targeted towards male readers, usually young men, so the ladies have to show their 'assets' as much as possible (and thankfully ALL characters that mashima created have depths and distinguished personalities)...it's shonen, fan service it's the way I suppose
Best female characters? of course Erza the Mery sue is an exellent character
why does a female character (mainly Lucy) has to go naked/topless every second chapter? it got really fucking annoying... i loved fairy tail because it has (imo) the best female characters when it comes to the shonen genre, but it gets on my nerves that mashima over sexualizes them