Irelia makes you have to manage your stun and all that, so I would've thought she was mechanically moderately difficult.
Riven on the other hand, oh boy. Mobility up the ass? Check. A delete button at Level 6? Check. Hell, if you get slightly fed as Riven your shield makes you tanky as hell, and did I mention her ability to delete anyone?
Ryze has that targeted root which kinda makes him extremely annoying to deal with, but they nerfed his passive really hard in terms of duration and you need to max your Q in order to get it even close to what it was before. His early game is better, but late-game he's been slightly nerfed overall.
Vladimir is a pretty safe pick as you have practically infinite sustain in lane if you don't get too harassed and late-game he becomes extremely hard to deal with if he's fed, since he has high HP and high damage output.
Cho'gath is another champ with high sustain but if you spam your Q too much then rip you because you run out of mana so quickly, I swear. However you do need to be good at csing in order for this to work. Also, I don't know if he's played top that much any more, I usually see him mid now.
Other top laners such as Darius and Sion may be quite safe picks for you if you think that you need some safety, and Garen is easy to play from what I remember, like, really easy. Your mileage may vary, of course, but there are quite a lot of viable top laners.