This is really a hard choice. Especially with Ritualists, I feel its even harder to choose a secondary.
What I did at first was went Rt/A just to unlock some PVP skills. Once I was happy with enough skills for my PVP assassin to play around with I made the swap. TBH it isn't a good combination so that was my only reasoning behind it.
Right now Im Rt/Mo because Im gone Resto Rit.... for damage though IMO Ranger, Necro and Ele are all good depending on what you want to accomplish. Necro for MM is a solid choice with Explosive growth combo. Ele with things like Spirit rift and Lightning Orb is a good call and Rangers with spirits or SQ could be an option.
Plenty of choices thats for sure. Have to really experiement and figure out whats best for you.