I've given this one a lot of thought. I think for Easton in the modern era, there are bunch of games that are defensible picks.
Honorable Mentions
I wouldn't take the 1991 Eastern Final against CB West, won by the Bucks 34-12. Easton and CB West were the #1 and #2 teams in the state and there was a lot of hype for the match up. But while the '91 squad was the best Easton team of the playoff era, and I think whoever won this game was going to win the final against Erie Cathedral Prep, Easton was just never in this game and it doesn't feel fair to flip the result, even if it gets Juan Gaddy a possible state championship as the greatest Easton player of the modern era.
It's also tempting to take the 1993 Eastern Final against CB West, won by the Bucks 6-3. This was a crazy game, with CB West scoring as time expired in the first half - they got stopped short of the goal line with the clock ticking, and hurried to the line and ran a QB sneak as the buzzer was sounding for the game's only touchdown. Easton turned it over in the red zone twice in the second half. If I get to wave a wand and make either of those a touchdown, Easton goes on to play North Hills, who needed a last minute score to beat CB West 15-14. All three teams were very evenly matched, but I don't take it because a win over CB West still gets Easton a coin flip game against Eric Kasperowicz, Lavar Arrington, and company.
The closest Easton's come to making it to Hersehy is the 2004 Eastern Final loss to Neshaminy, 21-14. Neshaminy broke a 14-14 tie with under a minute to play, Easton drove to the 8 yard line, and dropped the tying touchdown on the 2nd to last play of the game before throwing a desperation pick as time expired. Still, if I wave a magic wand and John Suozzo catches the TD and say Easton goes for 2 and gets it, reversing this result gets the Hersehy monkey off their back, but doesn't get them a title because 2004 Pittsburgh Central Catholic exists, and murdered this Neshaminy team and would have done something similar to Easton.
The 2000s version of the '91 CB West game is the 2003 Eastern Final against North Penn. North Penn was clearly the better team, though the 2003 Easton team was one of the three most talented Easton teams of the state playoff era. I think they could have beaten the '03 PCC team that North Penn ended up dispatching in the final, but that also is the underclassmen backbone of the '04 team that is one of the greatest of all time, so picking this one feels risky.
Easton's last great team is the 2014 squad that started 12-0, had a 2,100 yard, 42 TD season from Shane Simpson, and lost a brutally heartbreaking 13-10 overtime game to Parkland. A week later, Parkland lost 34-31 to St. Joseph's Prep, with D'Andre Swift scoring a long TD with four minutes left to play to beat the Trojans. Easton beat Parkland 28-21 during the season and held them to 86 yards of offense in the D11 title game loss. Easton had a pass tipped at the line and intercepted in overtime, then Parkland kicked a field goal immediately to win 13-10. Parkland's only touchdown came on a blocked punt in the first half, and they also booted a 45 yard field goal after a short Easton punt out of their own end zone. I think I very publicly melted down on this forum after that game. Magic wand wave those away, and Easton is an undefeated D11 champ that gets a crack at a St. Joseph's Prep team that Parkland gave everything they wanted. It also means that Steve Shiffert may have retired on top - the rumor was had they won he was planning to retire as Shane Simpson's career ended and he went to a final state playoff - rather than hang on an extra two years and have the ugly departure (which means maybe Easton hires Tim Moncman before he takes the Parkland job after 2015). But I don't think Easton matched up with SJP as well as Parkland did (mobile QB in Cross was the x-factor) and even if they pull that miracle, they still have to contend with Ben DiNucci's Pine-Richland team. Tempting for the Shiffert fallout, but no because I think this is the furthest from leading to a state title.
The Actual Choices
The most painful Easton loss since state playoffs started is the 2009 PIAA Quarterfinal against LaSalle, won 17-14 by the eventual state champs from the PCL. East goes up 14-0 on the first two plays of the game, four inches of snow fall during the game. LaSalle gets a long TD drive, then an Easton fumbled kickoff to tie it at 14. LaSalle leans on Easton and plays keep away in the second half, and hits the most clutch kick in state playoff history, a 37 yard field goal in the snow for the only points of the second half. Easton gets tackled from behind on the final play of the first half inside the LaSalle 20 on a breakaway shovel pass, and Easton throws an interception at the LaSalle 10 on their final drive of the game. Wave a magic wand and have Justin Souders not miss his block on the guy that eventually catches Quron Hughes on the shovel pass, have Q not fumble the kick that leads to the second LaSalle touchdown, etc. and Easton wins the game. While this seems like the least sure thing because it's only a quarterfinal - LaSalle is the eventual state champ, and easily beats Ridley (35-7) and State College (24-7) in their next two games to hoist the trophy. I feel confident Easton would have similar results against those two teams, and this was their best path to a state title if I can reverse the outcome of the LaSalle game.
The other choice is the 2001 D11 championship game against Bethlehem Catholic. The 2001 team is the second best Easton team of the state playoff era, after the aforementioned '91 squad. They gave up 4 points per game headed into this game, with the best rush defense we've ever had. Becahi, of course, had an All State and future FBS quarterback and four Division I receivers/tight ends and lit up Easton through the air. However, if I get to flip this result, Easton goes into the 2001 state playoff bracket that we've discussed here before is the most wide open of the playoff era - at least 7 teams I think were good enough to win a state title that year. And Easton's path would have been - Cumberland Valley, Neshaminy, Woodland Hills. Wing-T, power running attack, quarterback option attack. No team that threw the ball more than 12 times a game, and certainly not the spread offense that Bethlehem Catholic threw at them. A path that played perfectly to their strengths, and certainly not a superteam in the bunch like '04 PCC, '91 CB West, '03 North Penn. Becahi lost to Cumberland Valley because they couldn't contain the Wing-T, but Easton's coaching staff bottled up an even more explosive version of it two years later in a PIAA playoff game. Then Neshaminy and Woodland Hills are both two crazy physical, low scoring, old school football games. Flipping this result gets Easton on a great path to win the state title.
In the end, I pick the 2009 LaSalle game. I don't think that Easton team is the best of the ones I listed. But I feel the most confident in them beating Ridley and State College, versus CV, Neshaminy, and Woodland Hills, to get the Red Rovers the title missing from the trophy case.
Addendum for a Weird Historical Change
If I could fix any other weird historical quirk, I'd put the 1990 Easton team into the state playoff. They were D11 champs, but didn't have enough power points in the four-team playoff era to qualify for the 4A tournament over Wilson West Lawn and Ridley. But that Easton team owned a thorough 27-7 beating of 3A champ Bethlehem Catholic, and after starting 1-2, won 10 straight to finish the season. They were excellent - with the core of the '91 team (Juan Gaddy, Mark Libiano, Steve Billman, Dean Jones, Joey Jones) that is amongst the most talented in school history, plus FCS wideout and state 100 meter finalist Bob Apgar as a big play wide receiver. They could have given Ridley or North Allegheny problems if they got the chance, but they got locked out by the old format that only gave out 1 bid amongst all of D11, D2, D4, and D3.