Favorite from start to finish has to be the championship year, 07-08. That season had so much hype going into it and rightfully so, and unlike the first year of the Lebron/Wade/Bosh era in Miami, the Celtics lived up to the hype right from the get-go. It was perfect defensive, team basketball in every sense of the word.
A close second would probably be the '11-'12 season, with taking the Heat to 7 games when everyone, myself included, thought that'd series go 5 at best. It's also the last time we saw Rajon Rondo playing at an elite level, and I remember thinking that despite losing that series, that was his coming out party as an elite superstar point guard...oops.
My least faves are probably 05-06 and 06-07. There was nothing particularly interesting or special about that 2005 team and overall they were rather unlikeable. Guys like Tony Allen, Big Al, and Perk were still really raw and developing, Mark Blount and Ricky Davis were really starting to wear out their welcome, guys like Raef LaFrentz and Orien Greene were playing meaningful minutes, they only won 33 games...it was just an ugly season.
And then obviously for 06-07, an 18-game losing streak is never fun.