I think it's really tight between the Lakers, Spurs and Magic. Now that I'm writing, I'd actually change my vote to Lakers even though I voted for the Magic. My ultimate rank, using a 1-10 scale for each position, is
1. Lakers (Fisher 4.5-Kobe 10-Artest 7.5-Gasol 8-Bynum 6) 36
2. Magic (Nelson 7-Carter 7-Pietrus 5-Lewis 7-Howard 10) 36
3. Spurs (Parker 9-Mason 4-Jefferson 6-Duncan 9-McDyess 7) 35
4. Cavs (Mo 5.5-West 5-Lebron 10-Varejao(?) 6-Shaq 7) 33.5
(FYI: Celtics - Rondo 7-Allen 8-Pierce 9-Garnett 9-Perkins 7) 40
Note, that I ranked these guys in large part on how they perform in the most important situations - that's why Mo's ranking is relatively low and Fisher's relatively high. I also have to factor in age and injuries, which is why Duncan and Garnett only get 9s right now, and Nelson is at a 7 while I'd be more inclined to push him to 7.5 or 8 if I was sure about how he'd return. Mo's ranking also is a little low because I'm valuing that player at that position relative to the league. So Gasol would be a 9 as a center, in my opinion, but only an 8 at the stronger power forward position.
The Lakers have a very good starting unit with a weak but experienced and smart point, the best 2 guard in the NBA, one of the 5 best small forwards, one of the 5 best power forwards, and at the very least a legit center, top 12 in the NBA. With the potential of Bynum to step up this year and maybe Farmar to bounce back and take the starting role from an aging Fisher, I think they also have the best potential to be better than expected.
While I like Orlando's starting 5, I think the Magic have by far the weakest bench of any of the top 5 teams, which is why I'd ultimately put them at the bottom of the pack. I think the Spurs have the best bench of all 5 teams.
I think this is premature though because I have a feeling the Cavs will add at least one more starter, probably at the 2, pushing Delonte to his best role as a backup combo guard.
What's more, I think it should really be about the best 5, not the starting 5. For example, if you replace Mason with Ginobli in San Antonio's starting 5, they vault above the Magic and Lakers. Then again, if you replace Bynum with Odom (if LA keeps him), their potential goes down but their actual overall quality goes up because Odom and Gasol play so well together.
Overall, I'd rank those teams 1. LA, 2. SA, 3. Orlando, 4. Cleveland. But I think Cleveland will add a piece to move above the Magic, while the Magic will somewhat foolishly stand pat, relying on Redick, Anderson and Johnson for all their bench play. I think if San Antonio stays healthy this year (a bigger if than it is for Boston, given the major problems Ginobli and Duncan have had the past couple years and McDyess in much of his career), they'll be the best team of the remaining 4 and maybe better than Boston because of their great depth (Ginobli, Bonner, Finley, Blair, Hill, Udoka, probably one more big man) off the bench. If we add more bench pieces though, I think we retain the edge.