Out of the 4, I would say it would be Trundle.
To be an effective Rumble you need to be extremely good in managing heat, which can take time to get used to. Rumble also has the issue of having the click drag multi, much like Viktor e which is a pain.
Quinn currently has the issue of being a Melee ranged add with her ultimate. This makes timing key, but she's near useless without the enemy having a lack of cc. Imagine a Tryndamere without his ultimate and you got Quinn's ultimate.
Zac is kinda useless currently, the new jungle items don't really suit him, his damage is subpar, his sustain is a tab bit weak, and his ultimate is easy to cc. He does have his merits, including the best Zhonya freeze ever but to play him well requires a lot of practice.
Trundle is great because he has everything a top-laner needs. CC, Sustain, Damage, Splitpushing, Peel. He works great as counters to hyper tanks, with his ultimate making the precious Randuins as strong as a chain mail. His sustain docent even require you to auto minions so he's very forgiving if a trade goes wrong or someones poking you.
All 4 have their uses but i would say that Quinn and Zac require too much experience to be used effective game 1-50 and Rumbles ultimate and management of heat is a pain. Also nothings better then trundles dance.