eh, personal preference to a point, but here's my reasoning:
1. Hopefully you have a good player playing UB on your team, since he's so good if played right.
2. He is very powerful, but is easier to die with than say, Sedna.
3. Fewer other people will want Sedna, so as a noob you can be Sedna fairly consistently.
4. She has monks, learning how to play with them/hold lanes with monks is good practice for playing an assassin later (if you choose to).
Going UB would also be a fine choice, but I honestly advise him to go one of the top three generals first. But any of those four would be fine.
