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This is kind of a hard question to answer.
The best Shredder figures are definitely the various versions that NECA has released recently. Shredder was released in three convention exclusive sets, one based on the TMNT arcade game, one based on the original Mirage comics, and one based on the 80's cartoon. The problem is that these sets retailed for 100 to 200 dollars and probably go for much more now on the secondary market.
The best advice I can give you is try to find someone selling a Shredder figure on its own for a reasonable price. Or hell, maybe take your chances on a bootleg.