whos better jungler warwick, udyr, jax, yi, alistar, blitzcrank

Udyr, because he's useful from beginning to end. Yi snowballs hard, but a well-built team can shut him down easily. Warwick is a terrible ganker until 6, and provides little team utility. Jax takes too long to really get going although his ganks can be pretty good. Alistar is probably #2 because of his skillset, but doesn't do as much damage as Udyr. I've never seen a Blitz jungle. I can see the potential, but he would be used more if it was a good idea, I think.

But the real question is why you left out so many amazing junglers? Rammus is underplayed and unappreciated. Shyvana, Zac, Volibear. Those are some amazing junglers and there are many more.

But a good Udyr jungle is an amazing benefit to the team at all stages.

udyr is the strongest right now. blitz and ali aren't as viable. though perhaps with the new changes. jax needs a ton of farm. udyr needs to be mindful of his match up. he's very vulnerable to say jarvan if he doesn't run flash.

Udyr wins hands down. I agree with Amak though, there are better junglers than the ones you have listed. Remember though, skill and playstyle will have a large role in your success so you may have more success with one of the "weaker" junglers compared to other stronger junglers.