Udyr or Zac?

I feel like Zac might be better for you since he and Shen are both tanks teamfight disrupters. Udyr is more of a splitpusher

It really depends on what you're looking for? Shen's pretty nice on both disruption and split pushing, Zac and Udyr are just those to extremes. Udyr is a great splitpusher but his disruption is mediocre (unless you consider distraction as disruption, then Udyr's top tier for that) Zac is a great disruptor but his splitpushing is mediocre.

Udyr is a fairly fun champion imo. The only problem is that his only ranged damage (the 3rd AA effect from pheonix stance) is barely even ranged, and he has no real gapcloser except the burst MS from his E. Riot buffed the DoT effect on his Q a few patches ago, and it made it kind of retarded damage wise honestly. He has really great lane sustain in top and jungle due to his W, and can be built AD, AP, or a tank/bruiser, depending on how the game is going and your team comp.

I don't have any Zac experience at all though.