I want QFG 1-5, are the other games any good?
Let's see...
KQ1-2-3 are kinda classic, must-play I think. They are rife with the typical Sierra 80s rage-inducers (dead ends, random deaths, not-so-random deaths, and lots and lots of stairways where you WILL fall off because the view uses a shitty perspective and what you see as solid ground does not always match what's programmed as solid ground), but they've also got some great puzzles, and some that are among the toughest Sierra ever made (the guess-my-name one in original KQ1 is infamous). Of these I think 3 is the best, as its puzzle design is much more interesting, though KQ1 has the nonlinearity going for it (and it's probably the most nonlinear of the KQs).
KQ4-5-6 are in some ways very different. KQ4 is typical in terms of puzzles, though it was a landmark in many ways (female protagonist, one of the very first MIDI soundtracks and a very good one at that, huge graphic upgrade even though it was still EGA, and a MUCH improved parser over the AGI games, though nowhere near as good as Infocom's). KQ5 was shocking on release, it was probably the most beautiful game visually, but it has annoying and nonsensical linearity in puzzles, lots of mazes, lots of unnecessary dead ends, and all in all I think it's one of the weakest games in the series. Still not a bad game though. KQ6 is probably my favorite, probably the longest game, multiple paths at the endgame leading to different endings, lots of fun puzzles, some pretty good writing in parts, great atmosphere in parts (Isle of the Dead) and all in all it nails everything that makes KQ what it is (especially the fairytale atmosphere with a dash of mythology) and perfects it.
KQ7-8 are not the best. KQ7 is a Disneyfied version of KQ, so same fairytale but minus some of the harshness that made some of the other games more interesting (particularly 3 and 6), easymode puzzles for the most part (even for KQ). KQ8 has nothing to do with KQ, it's a first-person action-adventure game. Not a bad one, in fact I like it quite a bit, but if you're buying it because of the name then do keep in mind it's just a brand name stuck on an unrelated product.
The PQs are a mixed bag. PQ1's pretty fun, though it relies a little too heavily on the police procedures as puzzles, and all solving these requires is looking up the manual (it's nowhere near as bad as Codename Iceman in this respect though). PQ2 is supposed to be the better game, but honestly I really dislike it. It replaces the "look-up-the-manual" puzzles with trial and error ones, where you keep dying so you can figure out what needs to be done. It wouldn't be so bad if dying was actually fun (like in QFG or SQ), but this ain't no SQ. PQ3 is unfinished and it shows, but it's probably my favorite because it balances procedure and actual puzzle solving nicely, and it does require you pay attention to what the game is telling you to avoid some deaths (including a glorious troll one at the end that many newfags complain about as "impossible to solve without foreknowledge"). PQ4 is a shit game all around; it has some of the ugliest high-res graphics I've ever seen, really annoying hunt the pixel puzzles, railroads you into stupidity several times, and all in all the weakest entry in the series. Generally I don't think the PQ series is worth it unless you just want to play all Sierra games (and since half of the series is good, why not)
The SWATs have nothing to do with PQ other than name. SWAT1 is HORRIBLE, one of the worst FMV games to come out at the time (which says it all...), avoid at all costs. SWAT2 is an isometric tactical game, so if you like these you may like it, just don't expect a Sierra adventure.
SQ series is great from start to finish, so grab them all. Some entries are weaker than others; SQ2 relies a little too much on dead ends and random deaths, SQ4 is a little too easy and short (though in terms of humor, plot and especially atmosphere it's probably the best), SQ5 is a little uneven due to being made by one one of the Guys from Andromeda after they had a falling out, and SQ6 is also uneven due to being started by Josh Mandel then finished by the other Guy From Andromeda. None of the uneven ones are bad games though, and parts of both 5 and 6 are just as great as anything in the earlier ones. Entire series is highly recommended.
For remakes, pick and choose carefully. KQ1VGA is a perfect remake, though also a kinda pointless one. KQ2VGA is so different that I wouldn't touch it with a ten-foot pole, it's basically a fanfiction version of the original. Considering KQ2 plot and puzzles aren't exactly top of the line that may not be a bad thing, but then you're really playing a very different game. If you want to play KQ2VGA do so, but not as a replacement for the original. KQ3 has 2 remakes, one's a straight VGA remake, the other is a sequel to KQ2VGA which therefore also takes some liberties with the plot and puzzles (though not as much as KQ2VGA did). SQ2VGA is a more-or-less straight remake, would probably make some of the puzzles less rage-inducing, which may not be a bad thing since they're not SQ2's high point. QFG2VGA is a fantastic remake; mostly straight, with some optional writing that fits so well you would never know it's fan-added, a couple of optional bosses (which I avoid), and a combat system that's probably the best in the series. QFG2VGA is actually the only one I would heartily recommend over the original, unless you have some special attachment to the parser (it has to be said, the point and click does feel a little dumbed down compared to parser).
I don't think I need to mention QFG since you're already get them, but in case you were having any doubts -
GET THEM.