Not a huge fan of "Q", and I'm afraid I haven't seen Voyager beyond season 2, so with those provisos, my faves are:
"Q Who" - Q doesn't appear that often, maybe that's why he's not as annoying as usual. But a great episode.
"Tapestry" - the premise is familiar, but Stewart and De Lancie are so good together. Script is great.
"All Good Things" - again, less of a "Q" episode for me, but it's a great ep regardless.