Biggest and most important no doubt, but not sure it has a higher quality than the Champions League tbh although it is far more exciting.
Quality is subjective - I'd agree that the CL is better in terms of entertainment and even the cohesion of teams in an attacking sense (I imagine it to be much harder to get a team playing good football when the players have barely played together competitively and only have a month to get to know each other, compared to at club level where they spend years playing with each other every week). But I think the standards are higher, or at the very least on a par with the final stages of the CL, at the World Cup, especially beyond the group stage when the crappier teams from the weaker continents are knocked out, because every game is effectively a CL final for those players who've dreamt of this moment. This is the level at which national heroes tend to be made, and everyone will be, or try to be, at the absolute top of their game because of the expectations, the buildup, the profile, the status that can be achieved by playing well, etc etc.
On top of this, whilst there are better functioning teams at club level and arguably some better first teams, I think the top international squads have a depth that is rarely matched at club levels, especially in traditionally overlooked areas like defense. Most of the elite clubs in the CL actually have pretty thin squads in one area or another, e.g. Barcelona at their peak (arguably the best club side ever) playing with midfielders at the back due to their only two first choice centrebacks being injured. Alot of these World Cup squads are filled with experienced solid-to-great players in nearly every position, and there tends to be very few genuine weaknesses - injuries often aren't as debilitating at international level as they are at club level because of this depth. Just look at Spain's midfield squad for example, I doubt there has ever been a midfield squad at club level with that degree of depth as noone would have the budget for it or the ability to attract 9 players of that standard (and keep them happy). In addition to this, there are always some poorer nations where at club level they are unable to do anything, yet have national teams that are surprisingly exceptionally solid and far better than the "equivalent" CL teams (e.g. Uruguay, Mexico, Turkey, etc, compared to Ajax, Brugge, Panathinaikos, etc etc, in terms of CL sides from weaker nations). I'd view it as the World Cup having more "Simeone's Atletico Madrid" type teams than the CL does - it's very hard to perform against solid teams with a decent gameplan, a team with no weaknesses where everyone is trying their absolute best.
Whilst I appreciate there will be opposing views here and there are always exceptions (e.g. Miroslav Klose being a goalscoring machine in the World Cup), I also think the World Cup is a much harder stage for attacking players to perform in, especially the bigger names who are likely to get singled out and focused on. Alot of the genuinely great World Cup performances in recent times have been by centrebacks who've given their all, and many of the flops tend to be attacking players who are club level greats able to freely do what they want at domestic/CL level yet struggle to even get into the game on the World Cup stage. I think this has as much to do with the above two points made as it has to do with crap managers, tactics, roles, etc.
