ac1 is the only one worth skipping, if you dont play AC2 and ACB (in order) then dont bother getting revelations. not even for multiplayer (which has roots in the storyline as well)
AC1 lays the foundation for the overarcing story but the majority is about altair. the major plot stuff doesnt start showing up until the end altho its slightly hinted at at around the middle. the mission structure of AC1 is completely different from the other games in the series which is what the "repetitive" comments are referring to
in AC2 the story literally explodes, tons of different story arcs appear and you play as ezio. probably the best game in the series. you HAVE to play this one
ACB the story focuses more on the overarcing storyline but still continues with the ezio story, you most likely want to beat this b/c theres a HUGE FUCKING TWIST at the end...