While Garen is fun, it depends on your play style.
Ori focuses on team play with small chances to 1v1, but she really shines in team fights with her massive amount of AOE and burst. She has a higher skill level than most other mids because you have to micromanage your ball's location in fights to maximize your potential. Timing, zoning, and mana management are also important with her. Though you have strong autos with her passive.
Syndra is more single target but can do a lot in fights with her AOE stun and high damage. She has a lot of 1v1 potential with her ult and combo. Plus she has hard CC before 6. Farming is easy with her Q spam and she can deny ganks with her stun and poke from distance with her combo. You have mana management to think about too.
Basically, Syndra is my suggestion but it really depends on your playstyle.
Could always play Karthus.