I agree that Sierra went into a slump in the early to mid nineties (from which it obviously never recovered). However, I reckon that games like KQ5 and SQ4 represent the peaking in quality with regards to Sierra’s output before the decline set in.
Though I personally reckon SQ3 to be the best of that series, I do recognise SQ4 as a close second. And it had (and still has) simply marvellous graphics and sound! KQ5 in turn, I believe, is the best game ever to come out of Sierra. It had everything that made/makes point-and-click adventure games so enjoyable, including a rousing sense of adventure that none of the other KQ’s are able to match, attractive graphics that have stood the test of time, varied settings, unforgettable characters, etc., etc.