He was the best player on the 02-03 Senators that were a couple of bounces away from going to the Finals. We can't be sure, but it was commonly accepted that the ECF was the defacto cup finals that year, as the winner of that series was expected to roll over the Ducks (Devils ended up winning in a closer series than anticipated).
So he was as close as possible to being the "best player on a Cup winner" here in Ottawa. He was always a superstar level player in Atlanta, it's not his fault he was surrounded by a heaping pile of garbage. By the time he started chasing cups it's clear he'd taken a bit of a step back, but that doesn't take away his peak.
Do you know how many goals Hossa scored in the 2003 East final? Zero. He did manage 4 assists, but for a 45 goal scorer during the regular season, that's quite the lackluster showing. He was hardly alone in that regard, but still...
And as always with hypotheticals: coulda, woulda, shoulda, didn't-a. And given how the Devils struggled with the Ducks, who's to say the Sens wouldn't have lost? Especially with Gigure en route to a Conn Smythe Trophy. While Lalime was actually solid (outside of one particular game) in the playoffs, I know who I would have bet on in that goaltending duel, and it ain't Patrick.
In the 2004 playoffs, outside of game 1 where he notched a pair of goals, he managed just 1 goal and 1 assist during the next 6 games. Everyone will point to that one game where he had around 10 shots, but IIRC a lot of those were distanced and unscreened. The way Belfour was playing, that wasn't going to cut it.
Hossa was a star, no question, and there's a serious case for him being HHoF worthy. However, Sens fans really need to stop holding on to the notion that keeping Hossa would have resulted in the team winning the Cup. I mean, it's a hypothetical, so we'll never know, but looking at his playoff production, I don't see him being the guy to lead the way. As I said in another post, the only time he produced like a superstar in the post-season was when Crosby and Malkin were the ones getting most of the attention from the opposition. And in Chicago, several other guys all played bigger roles each of the 3 times they won.