Most words can be decoded. Many words taught as "sight words" are just high frequency words like "said", "for", and "they". It's better to give kids a strong, solid strategy to decode all but the few true sight words. I don't teach any sight words directly. I only address them after students are dependent on the decoding strategies I teach them. They need to be proficiently reading CVC, VCC, CCVC, and CVCC words first.
Here's a short list of words that I allow to be memorized as sight words: one, two, of, does, and who. What other sight words can you think of that can't be decoded with the right background knowledge? In which words can you not match sounds to letters in the order they come in the word? (ex: one)