I also think, that Argentina is not the strongest team of the remaining for nations that are still in this world cup. For me France is the big favorite. As you already said, Croatia is a very strong team too. They have a lot of quality in their squad and they also play with a lot of passion and conviction. Their greatest strength in my opinion, however, is that they are very good at defending and even if the opponent is making massive pressure towards their goal then the croatians still remain calm and are always defending well.
If Argentina wants to go through tonight, then they will need more than just Messi. They will need a good team performance.
Croatia and Morocco are the teams that rely on defense, while Argentina and France are representatives of the attacking style, so we have to see who will be stronger. I think that Croatia and Agrantina are very equal teams, if in the match with Brazil, Brazil was the clear favorite, but in this match the chances are equal. the Croats can score today, but can the Argentines score against the Hovats today?
I wouldn't say for the Croatia team that they do rely on defense, for critically looking back at the game between Croatia against Brazil you will observe that Croatia had more control of the midfield than the Brazilian team did and that was what made them able to always absorb every pressure pulled on them by Brazil in that game. Today's game between Argentina and Croatia am quite sure both teams would be very careful especially Argentina in their attacking style and rather be clinical as counter attack is possible.
As for the Moroccan team they have been very successful with their defensive tactics, and that's because they have a very strong defense line which has proven itself by not conceding a single goal from any of their opponents except for an own goal they had. But it doesn't look like their defense can withstand the sting of the France team come tommorow except the unexpected happens out of luck.