As far as ability and will goes... Hal Jordan. Kyle Rayner is a close second just because he had no formal training but as still held his ground with some pretty tough bad guys.
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It may sound stupid but he would beat any other green lantern probably. After all they could toss stuff at him but he would be able to do that AND attack them directly.
__________________ You probably think that just because I'm using the internet at this time that I'm doing nothing. Well the washing up isn't nothing, and I'll be doing that any bloody moment.
How exactly could Kyle beat Hal? The rings strength goes by will power and Hal has the strongest will out of all GLs. Its true that having an immunity to the color yellow is an advantage of Kyle's ring but he has never gone under the training that Hal has gone through. The main argument that Kyle would have on his side is that his ring holds more of a charge than Hals does. Kyle's ring will not run out of power after 24 hour providing he does not use all of his energy. But it is possible for Kyle to use all of his energy within a 24 hour time period and need to recharge. Where other GL rings are limitless power within a 24 hour time frame but are rendered drain at 24 hours besides the reserve it has. The other main difference between the two is that Kyles ring does not hold a reserve. Once he is out he is out... and he has to will his ring to put a forcefield around him if he enters space or a harsh environment where normal GL rings activate that force field automatically.
Since the poll is "greatest" GL, mine is Alan...I just think he looks the coolest for some reason. Can't really say would would win in a fight, but that's beside the point...