At this point I seem to get my best shaves from a Merkur 39C slant. It took some experience to become ready to get the best from it, so for someone new I would vaunt my Mühle R106. If I were on a $20 budget, I'd say the Long Handle Perfecto. Despite the request to specify "only one", that "one" could vary with the circumstances.
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Check out the Wiki, lots of good information there! Pretty sure I've ran across this question many times while researching wet shaving on this forum. Definitely a YMMV!!