I just can't get there with Messi. And it's strange really.. of all the great players of the last 20 years, I have watched Messi more than any other (I'm not including Bale as a great great player even though he's phenomenally talented). But at the end of the day, what sets Messi apart is that he hasn't won anything for his country. I can't get past that. His lack of international production is almost Dimitrovian. No one doubts his talent, and he gets to advertise it playing for one of the greatest club teams we've ever seen. Even Cristiano has won a national title
This is my thing about Messi and the national team argument (or World Cup argument). I'll actually start with the World Cup argument, which in fairness, you haven't brought up, but I'll discuss anyway because many use it as a point of criticism towards Messi (ie. "How can you be the best if you haven't won the World Cup?").
The World Cup takes place once every 4 years, for a period of one month. Which means that A LOT has to go right for you to win it. It starts with the national team coach, the talent level around you, injuries, refereeing, luck, the path to the final, etc... We're talking about defining a player's whole career because of an event which lasts 1 month and happens every 4 years. To me, that is a bit absurd.
Nowadays, I don't think there's even an argument that most put emphasis on club football in terms of focus throughout their careers, which is why you rarely see the absolute best managers in the world coaching the national team. In turn, this meant that the state of international football is putrid (seriously, watch a Champions League match, and compare it to some of the matches we've seen at the last Euro championships. It's night and day).
So the whole World Cup argument is easily countered by the following: When Messi does what he does in the Champions League, he's doing it on a higher level. Yes, a higher level. Barcelona, Real Madrid, Juventus, Bayern, etc... are much, much better than any national team in the world. They get to train together every week, play every week (often twice a week), drill tactics, gameplans, develop players, etc... Yes, there is no doubt Messi plays on an absolutely fantastic team (and that is a major point for Maradona, who did what he did for mediocre Napoli at the peak of Serie A, no question), but this fantastic team wouldn't have won anything without him. There's no question about that.
We are talking about a player who week in and week out, since pretty much 2007, has been playing a level of football which we had never seen before, literally every week. He doesn't go through a bad stretch (a Messi drop in form means going 2 weeks without scoring), and he does it all. He's the best dribbler in history (only Maradona is in the conversation with him in that regard), the best finisher with history (I'd maybe put Brazilian Ronaldo ahead of him because of his ability to score with both feet), has the best first touch I have ever seen, is an absolutely phenomenal passer, etc...
Think about that, dribbling, control, finishing and passing are the most important aspects of football. And he is exceptional at all of them. He does them so well nobody actually cares or notices that he doesn't use his weaker foot all that much (just like Diego) and doesn't score with his head often.
Guardiola said something about Messi that is absolutely true... it's not just about the obvious things he does (ie beating 3 players and scoring), it's about the simple things. He never loses the ball. The simple one-touch passes when he needs to. Never fails to complete a pass. Never makes the wrong decision. For a player with such inhuman quality and talent, it would be very easy for him to try and do too much every time, because he can (Neymar often falls victim of that). Messi just always makes the right choice. I've never seen a player whose chances at scoring from just outside the box are the same as scoring from a clear one-on-one with the keeper.
I always use Zidane as a highlight of how good Messi is. Zidane is one of the best I've ever seen. Absolutely phenomenal player. He's not even close to Messi's level. That's how good Messi is.
As far as the national team goes, he's reached 3 finals in 3 years (World Cup in 2014, Copa American in 2015 and Copa America in 2016). He was crowned as the best player of the tournament in two of them (World Cup and Copa America 2016). What more does he need to do? I guess the obvious answer to that question is: win the damn thing. And that's fair, to an extent, but this is a team sport. We can't lose sight of that. Argentina would probably have won the World Cup if Di Maria, who was sensational throughout the tournament, didn't miss the final due to injury (as did Aguero).
Gonzalo Higuain and Rodrigo Palacio missed absolute sitters in the 2014 World Cup final. If they score we're talking about Messi as a World Cup winner. Think about that, a player's legacy is defined by very two specific events he had no control over (Higuain had similar insanely bad misses in both Copa America finals). Meanwhile, Buruchaga scored in the 1986 World Cup final and cemented Maradona's legacy (Diego was out of this world in that World Cup but he didn't have a very good final). Cristiano Ronaldo is lauded for winning the Euros yet he was taken off injured 15 minutes into the final, and pretty much won it because Gignac had an awful miss for France late in the match and Eder scored for Portugal in extra time... Again, two events Cristiano had zero control over. He wasn't even on the pitch.
I just can't attribute more importance to national team "failures" than the champions league heroics, the Liga dominance, the big match performances in Europe, etc... that Messi displays every season. Over the last 8 years, he's scored the most goals AND produced the most assists out of any player in the world. That is absolutely insane.