Champions are indestructible tanks and have probaly been around the longest and their design is virtually unchanged. I've owned one for at least 25 years and I bought it used then. When I went to buy a replacement part on their website I gave them the serial # they told me mine was built in 1959!! Still going strong and will last another 40 years.
Having said the above and having used one all these years they are very fast with hard veggies but they don't do well with anything soft and get very little juice from leafy greens. There is also the issue that because of their fast spinning rate they introduce heat and oxygen into the juice speeding up the oxydating process so the juice must be drunk immediately and it will not save for long. I have also owned centrifugals like the Acme and they too are quick but they don't do well with leafy greens either. These 2 types are probably the oldest outside of the very expensive presses like the Norwalk press. They are the best but as I said very expensive and take lots of work.
For the last 3 or 4 years I have owned an auger type Omega 2003 http://curezone.com/forums/fm.asp?i=1657306#i which I bought used for half the new price. Bulletin boards at healthfood stores are usually good sources and there is always Craigs list. The auger types are the best modern juicers and in my opinion they make the best highest quality juice. Since they are slow they don't start the oxydation process as quickly and you can actually makes batches that you can refrigerate. They can even do wheat grass that neither the Champion nor the centrifugals can do. Their only drawback is that they are slower and you hafta cut your veggies comparatively small.
Jump to the latest technology The Omega Vert which is a new kind of auger type juicer with a wide opening and you don't hafta chop thing so small and they work relatively fast. From all the reviews, testimonials and YouTube demos it's the one that I am currently lusting after. Since they are new to the market I haven't found any used ones.