Game 1, if only because it would be really difficult to win 4 in a row (see Eastern Conference Finals, 2010). Throw a punch in game 1, take a punch in game 2, and then throw out a barrage of haymakers in games 3, 4 and 5 and get the KO.
I actually don't believe the Celtics need to win 4 of the first 5 to win the series - they've proven perfectly capable of going back on the road to win either a game 6 or 7, as needed. BUT, that wouldn't be the preferred way of doing it, I don't think.