My favorite episodes of TOS are CITY ON THE EDGE OF FOREVER, THE MENAGERIE, THE SQUIRE OF GOTHOS, SPACE SEED, WHO MOURNS FOR ADONAIS?, THE ENTERPRISE INCIDENT and THE EMPATH. There are none so poor that I'd actively avoid them, though some, such as THE WAY TO EDEN, are admittedly pretty silly.

As far as TNG, in reviewing the series over the past few months via TV reruns I've come to the conclusion that approximately 15% of its episodes are superlative, another 30% are moderately enjoyable, a further 30% are mediocre and the remaining 25% irredeemably dismal. Not a great average, and yet CHAIN OF COMMAND, THE BEST OF BOTH WORLDS, THE MEASURE OF A MAN, ELEMENTARY MY DEAR DATA, CONSPIRACY, HOLLOW PURSUITS, RELICS, FRAME OF MIND and GENESIS rank among the very best the entire TREK canon has to offer. 

Never saw much of DS9 but enjoyed the Ferengi episodes (the one set around the Roswell Incident was especially cute). Have almost no interest in the remaining series.