Asking "what's the best juicer" is kind of like asking "what's the best tool." It all depends what task you need done. You can tighten a bolt with a pliers, but it's not really the right tool for the job. By the same token, each juicer has its strengths and weaknesses.
Single-screw machines like Samson do a great job with greens and are easy to clean, but are of limited use for carrots and other hard veggies. Champions do a good job with carrots, but not such a good job with wheatgrass and leafy greens. Greenstars foam greens too much for my taste, although a poster just below finds that he can control the foaming by oiling the gears with olive oil. They do a good job with carrots, but require considerable downward force on the carrots.
I could go on and on. You get the picture -- there's no best or worst juicer, just different juicers with different strengths and weaknesses. Some are better at juicing one kind of vegetable, others shine in another category.