I would try Christie and Ayane for starters. Make sure you know the rock, paper, scissors fighting mechanics in this game and you should be fine. --- "Relax and let me do all the work," says Morrigan Aensland to Ghost Rider.
Not sure about now but back when I played regularly, the most used characters online based on my own personal experience were Mila and Hitomi for the females and Rig and Ryu for the males.
As for the easiest and hardest characters to use question, now I'm no expert by any means as this was my first DOA game, but again from my own experience Hitomi and Rig were fairly easy to use while the VF characters were hard.