Datsyuk was for sure the "better #13". Sundin may have been better offensively, but Datsyuk was way better defensively AND still had many 80+ points seasons, plus 2 Cups, and 3 Selkes. He was dominant.
I wouldn't even put Sundin at 2nd best. Selanne should have that, even though he switched back/forth bewteen #13 and #8. He was SUPER dominant offensively, and won a Calder, a Rocket, and a cup.
Sundin should sit comfortably at 3rd.
Look, I'm not trying to take away any of Datsyuk's accomplishments as he was a fantastic player, but I think there is a real discussion or debate that could be had in terms of the quality of players, especially when factoring in teams and level of support.
During Sundin's tenure as a Leaf, could you ever say there was a year where he was not the best player on the team? Mogilny slightly outscored him in '03, but even then I wouldn't argue that Mogilny was better or more valuable to the team. I don't think there is any debate that Sundin was certifiably the best player on the team all of those seasons.
With Datsyuk, was he the best player on his team every season? Arguable. His first few seasons, he had Yzerman, Hull, Shanahan, Fedorov, Lidstrom etc who were still all stars at that time. Post-lockout he reached elite levels, but even during the Cup run years ('08,'09), you could argue that Zetterberg or Lidstrom were just as valuable or better. Zetterberg himself was nominated for a Selke in 2008, played both wing and centre and was an incredible playoff performer, winning a Smythe while shutting down Crosby and almost doing it again in 2009. Datsyuk's playoff numbers pale in comparison to his. Then you have Lidstrom, who I don't think I need to speak for really...and they had Marian Hossa for their 2009 run as well.
As a a Leaf, did Sundin ever have anyone close to a Zetterberg or a Lidstrom? Hell, did he have anything close to even one season of Marian Hossa, or Brian Rafalski? The best teammate Mats ever had in the NHL was Joe Sakic on the Nordiques. Sakic is also unanimously considered to be better than Mats, yet when the two played together, Mats was actually outscoring him by his third season. If the Nordiques never traded Sundin, and proceeded to still get Forsberg they would've been a dynasty and Sundin is probably held in much higher regard overall.