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my favourite non-pentax lens is Sigma 50-150/2.8 HSM II. I was split between this one and DA*50-135. While I was just charmed with optical quality of the DA*, the SDM performance (coupled with my K200d) simply annoyed me... Oh, and the sigma cost me exactly half the price of DA*.
Before I bought 50-150, my favourite non-pentax was sigma 10-20/4-5.6.
cheers, Luka