I'd have to say the mid 2000 Suns only because of all of the implications a Suns title would have had for the NBA.
At the time, the conventional wisdom was that a team built in their mold would never be able to win the NBA Championship.
The 05 Suns lost Joe Johnson to injury in the Conference Finals. The 06 Suns lost Amar'e for the season and still made the Conference Finals. The 07 Suns lost Amar'e and Diaw to pretty weak suspensions and would've easily beat the Jazz and Cavs. Eventually, the whole team was dismantled for cap / salary purposes.
If any one of those teams were able to break through and overcome that bad luck, the entire NBA landscape would've changed. I may be overdramatizing this, but it would've made an entire brand of offense and style of team legitimate. All of the other teams on this list can't say that.
The Pacers, Sonics, Jazz, Knicks, Bucks, Kings, Blazers were all just fairly conventional good teams who never broke through. Rick Adelman coached two of those squads, worth note.