In my lifetime:
1994 - I was 3 years old. My older brother says that I watched it and liked Brazil, but I remember absolutely nothing, so I can't comment.
1998 - watched every match, my favorite was Argentina with Batistuta. Fond memories, but I think that I was so young to really be able to judge the quality.
2002 - watched every match, and I think it was a bit shit. Brazil was the only top team (maybe followed by England), my Argies crashed early, and South Korea were buying referees.
2006 - best in my lifetime. Italy, France, Argentina, Germany, Portugal, England, Holland and Brazil all had top quality teams stacked with world class player. A top team went out only when it faced an another top team. Italy eliminated France and Germany, France eliminated Portugal and Brazil, Germany eliminated Argentina, Portugal eliminated Holland and England. All matches when top teams faced each other were extremely competitive. So many fantastic players: Ronaldinho, Ronaldo, Kaka, Adriano, Zidane, Henry, Riquelme, Italian's impenetrable defense, C. Ronaldo, Figo, young Messi, Raul, Shevchenko, young Xavi and Iniesta, Lampard, Beckham, Gerrard, Rooney, Ballack, RvN, etc etc. I think that the list of superstars was higher than ever as was the quality of matches. So many classics: the battle between Portugal and Holland, Portugal vs England, Germany vs Argentina, France vs Brazil, Italy vs Germany, Italy vs France and so on. And what a dramatic ending it was.
2010: pure shit. Atmosphere looked awful, fecking vuvuzelas, Holland playing anti-football, Spain boring everyone to death and winning the world cup by scoring only 8 goals. A young Germany team was the only inspiring thing in that world cup. And Ghana, loved them to bits but it wasn't written for them to reach the semis.
2014: quite an improvement, but nowhere as good as 2006. Germany was decent, Argentina was okayish but struggled to score, Holland were meh, and Brazil was so bad. The other teams aren't even worthy to be mentioned. Only classic is the famous 7-1.
2018: the anti-2006, but still very good. If 2006 was all about top teams and classics, 2018 is all about surprises. Every game is really unpredictable, any team can defeat any other team, top teams lose from nobodies etc etc. The quality of football might not be as high as in 2006, but the excitement is as big, and every match is interesting.
If I had to rank them, I would say:
2006 > 1998* = 2018 > 2014 = 2002 > 2010.
* might be just nostalgia.