I can't comment on whether it's the 'best' or not, but Seaclear is free and seems to work well. It's not the easiest program to get started with, loading charts can be difficult at first, but it (and the charts) is free, it's very fully featured, and it does work.
You can get it here:
SeaClear GPS Navigation Software
Deep Blue C, 2002 C&C 110
Mt. Sinai, NY
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