IMO, top 5 biggest games in franchise history:
#1: Game 5 vs Sabres, 2007: Obvious. First time they went to the cup finals. They beat a rival who had beat them the previous year. A rival they also had a huge line brawl with earlier that season. Also, Alfie scored the goal, for redemption for the bad series goal he let in the previous season. That game, and that playoff run elevated Alfie from being a playoff choker to being the patron saint of the franchise.
#2: Game 7 vs Devils, 2003: Closest they've come to winning the cup in many people's opinion. They had a solid shot a beating the Ducks in the finals. They were an amazing team top to bottom. Presidents trophy winner. IIRC, Nieuendyk said it was the toughest team/series he ever played against.
#3: Game 7 vs Penguins, 2017: Tough loss against a borderline dynasty. One of the few 2OT game 7's in NHL history. It marked the start of the end for that core of players, and the beginning of the current **** era.
#4: 8-0 win vs Leafs, Oct. 29, 2005: This game was a huge turning point for the franchise. At that point, the young Sens franchise were forever under the thumb of the Leafs, who would often lose to the Sens in the regular season, but would beat the Sens in the playoffs because "they were tougher" and the Sens were "too soft". The Sens not only demolished the Leafs 8-0, but they beat the **** out of them too. McGrattan TKOed Tie Domi, something the Sens were never able to do. IMO, this marked the beginning of the Bryan Murray "
take no ****s" era, the end of the Jacques Martin "soft" era. It also marked the beginning of the Pizza line, and the end of the Battle of Ontario (it was never the same after that). The Sens also went of to crush the Leafs 8-2 a couple weeks later just to cement things further.
#5: 1-0 win vs Sabres, April 12, 1997: Last game of the season versus the top team in the division. Duchene goal. Sens clinch the playoffs for the first time.