I can give you an unbiased opinion because I do not work for any of those companies, but I have in the past! My personal favorite (and the easiest to sell) was Avon. Honestly, I thought Mary Kay and Arbonne were too expensive and the cost was a big turn-off to people. I didn't think their products were any better than Avon. I did love Melaleuca products, but they are pretty expensive also. One thing I liked about Melaleuca was the concept. You didn't sell; you bought. You could sell if you wanted to but the main idea was to get people signed on underneath you to buy their products also, then you'd make money off your downline. I loved the concept but never got anyone else interested although their products were wonderful.
With Avon I never had to try to sign people on, I never had to have home parties (which I hate), and I never had to pressure anyone to buy it. When people heard I was a rep they'd BEG me to bring them a catalog! Also, it's cheap to sign on (like $10 or $20) and with that you get a LOT of stuff; catalogs, samples, etc. You can earn up to 50%, which to me is really amazing considering a lot of their cosmetics are cheaper than if you were to go into Walmart and buy Loreal, Maybelline, etc.
Avon reps are always in demand, too. (At least here they are!) I know here in my town it's hard to find one. I'm looking for one myself, so if anyone in the Christian County area sells it, please PM me. :)
Good luck with whatever you decide!