He's not even the best goalscorer in his generation, let alone all time. Messi has got more league top goal-scorer awards, Golden Shoes, a better World Cup goalscoring record, a higher goal ratio in both league and Champions League per game, and a bunch of unbeatable records like scoring 91 goals in a calendar year, having the most goals in top-5 European leagues, and the most goals ever in a single league season. Loads of people think Messi is the better goalscorer, hands down.
Comparing him to Gerd Muller is just absurd. Muller was the top goalscorer in the 1970 World Cup with 10 goals, scoring in every knockout stage match and netting the winning goals in the 1974 World Cup final and semi-final. In Euro 72, Muller scored twice in both the semi-finals and finals, leading Germany to victory with the highest goal tally. These are achievements Ronaldo can only dream of. He was a 4-time CL top goal-scorer and had tons of legendary records like most goals in a calendar year, most goals in WC etc.
Also, Muller played in just 2 World Cups and one Euro having huge influence on each tournament, while Ronaldo's been in the game for 5 Euros and 5 World Cups regularly underperforming. Muller's scoring rate with the national team is 1.1 goals per game, literally double Ronaldo's. Plus, Muller didn't get the chance to face teams like Liechtenstein, Azerbaijan, Andorra, Faroe, Kazakhstan – you know, Ronaldo's favorite opponents – because these teams weren't around in Muller's time.
You can't become the greatest goalscorer of all time with 0 knockout goals in in 5 World Cups and a total of 3 goals in 19 knockout games in 5 Euros and 5 World Cups. These are the stats of an all time flop.
Not to mention, that supposed "greatest goalscorer of all time" could only win league top goalscorer award once in the last 9 years. In the same period, Messi finished the league 5-times as the top goalscorer.