making a assassin character evil and i am not sure who to side with i was thinking legion or yes man not sure what do you think

No one.
When I create my assassin character I'm not going to side with anyone.
By that I mean, there won't be any ending sliders for her.
After she's killed all major leaders or people with ranks then it's done, over, finito.

so far, legion seems to be the most interesting faction, however, they don't have as many quests.

even if you side with legion, you can complete most of the NCR and BoS quests. if they don't allow you to approach, just put on an NCR disguise. I've killed lots of NCR and they still allow me to approach without disguise, however.