Obviously, much of this comes down to your own cultural and individual expectations. As a bilingual native French/Flemish speaker also fluent in Italian, now splitting time between the US and Italy (and thus doing far too much flying), I have a possibly distinct experience on DL, KL, AF and even VS (not to mention AZ, although they are barely in and likely soon leaving SkyTeam). FWIW, my thoughts...
DL: excellent operational reliability but highly variable otherwise. Acceptable-good hard product (767-400, 330-200/-300), good-very good (330-neo, 350), terrible-avoid-at-all-cost (767-300); soft product is a mixed bag, often depending on crew and station - better quality food ex-Europe and from SEA, hit-or-miss from JFK/BOS, pretty dire ex-ATL/MSP/DTW, wine is pretty dire no matter where you depart; crews are also variable - 50% chance of cabin staff that doesn't care (esp. ex-JFK) or is a few cards short of a deck, 50% chance of really stellar crew and service. Hubs in SEA, LAX and ATL function well, acceptable at DTW and MSP, JFK-T4. Lounges are pretty terrible and definitely the worst of the pack, perhaps only matched by KL.
AF: solid operations and generally a better experience. Good all around hard product (I see very little difference in the new/refreshed 773, 772 and 359 cabins vis-à-vis prior "best"; have oddly yet to be on their 787-9); soft product is much better than others, both in terms of wine and food; service is generally good to outstanding, but this may be as a Francophone (I notice some crew treat non-French speakers differently, although far from the majority), although had a truly dismal crew on a mid-December JFK-CDG flight (these things happen and it was more than compensated by an outstanding one a few weeks later); lounges are very good, but only at CDG; I know CDG like my pocket and can do a 30 minute connect from Schengen to non-Schengen blindfolded after no sleep and too much champagne, that said that it can be a large morass for many (pro tip: follow the signs like a rat in a maze and ask for help from staff with tight connections). Word of warning - in/out of JFK they use Terminal 1, which is among the worst anywhere in the world (and I've seen quite a lot on all five continents). TBF, I don't think it's AF's fault, but still best avoided.
KL: bottom of the barrel on all counts and I will do whatever I can to avoid them. Hard product is a gamble, as have said others (787-1 is good, 787-9 ok, 772/773 horrible, avoid the 330); soft product reflects the food "culture" of the Netherlands, i.e. one that makes the UK look sophisticated - if you can wash it down with some (probably cheap and Australian) wine, you're lucky; service is... Dutch, a bit robotically professional but also not as variable as DL, being a native Flemish speaker doesn't help and indeed has led to being talked down to on more than one occasion (I just use English with them now); AMS used to be efficient but has gone into a nosedive and now seems to be worse than FRA - hardly a compliment; KL lounges are pretty dismal and nearly always overcrowded, zoo-like and anything but restful.
VS: small and quirky by comparison, but can be worth it if schedules align. Hard product is VERY variable (I actually like the old 330-300s and 787-9s for the lack of coffin-like beds, but these are not to many folks' liking; the new 330-900 is quite nice and 350s are okay); soft product is pretty good, although I have found the food hit-or-miss; service is one of their strong points and I've never had anything but good-excellent cabin crew; LHR is definitely not for everyone and they don't do much for onward connections in Europe, except to other SkyTeam hubs via partners (which is not terribly seamless); some of the best lounges anywhere and it's nice to have the arrivals thing in LHR.
AZ (not that this was asked for, but since we're on the topic): a basket case that may improve with LH takeover, but honestly I won't bet on it. Hard product the result of random acquisitions and takeovers, small fleet and lots of substitutions, so it doesn't really matter what you sign up for in terms of layout/aircraft - top this with a lack of reliability and you understand why anyone with choices avoids them for long haul; soft product is actually decent on long haul and on-par with AF within Europe, they do tend to have good wine on long flights; service is actually better now, oddly, but again it probably helps to speak Italian; FCO is a great connection point and doesn't deserve its reputation (much improved with the new terminal); lounges are actually not bad in FCO, all others are contracted out.