Out of the (not very many) baseball cards I have, I'd put my 1934 Goudey of Lefty Grove at the top. Out of the sets out there, from which I have only the Mel Ott, I am a huge fan of the very cartoonish 1934-1936 Diamond Stars.
Not my card, but look at this wonderful example.
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And not baseball at all, but I am a fan of Liebig trade cards. Between 1872 and 2001, they put out over 1800 different sets. Nearly all have wonderful artwork, and the range of topics is amazing. Here is one from 1906 (Astronomers, Series 1) that I have 4 of the 6 cards from.
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