When playing for Sweden, Sundin was the big star along with Peter Forsberg. Those two were considered a step above the other Swedish forwards/players. Please note, that Sundin is considered (at least) Forsberg's equal in that regard.
In international tournaments Sundin was as great as anyone. Among all the best players, he was voted into the tournament All Star Team in both Canada Cup 1991 and its successor World Cup in 1996.
He also made the All Star Team in the 2002 Olympics, where he also led the tournament in points, despite just playing 4 games. He was great in that tournament, and also was our best forward in the dreadful quarter final which we lost to Belarus.
Twice he was voted as best forward in the World Championship (1992 and 2003).
He also captained the Swedish team that won the 2006 Olympic gold medal, and assisted on the tournament winning goal.
Again, note the above accomplishments. Sundin was (at least) Forsberg's equal. (Other guys were considered steps behind. Those includes Alfredsson and Lidstrom. Renberg was even further behind.)
In the NHL, a 21 year old Sundin in his third season outscored 23 year old Joe Sakic (who would again outscore Sundin in the next season).
He then captained Toronto for many years and usually was their leading scorer, often leading in goals and assists.
In the playoffs, he continued to produce at about the same pace as in the regular season.
However, in the NHL Sundin always appeared a level behind Forsberg and didn't reach Forsberg's high scoring finishes or team success.
One thing used against him is that he had a season where he was outscored by teammate Mogilny.
So how do we summarize?
Great player able to step up in big games. A great and reliable leader.
Has proven he could captain and lead his teams to gold medals.
Didn't have great team success in the NHL, only had two top-ten scoring finishes, and was probably considered a step below the best players.
Speculation...
In my opinion he could as well have ended up winning Stanley Cup(s), as a captain, and scoring big goals on the way.
If swapping Sundin and Sakic, Sundin might have had stats and team success similar to Sakic, or possibly yet still be a step behind.
Might have had more success as a winger?
(By the way, Sundin was considered a betrayer when he as a teenager left Sweden for the NHL (at the time that was very uncommon), but quickly became forgiven.)