Anyone interested in started a bleach power level chart? I've seen a few attempts around the internet and they are terrible. I understand slapping power levels into bleach is harder than the DBZ/GT series because there are no definite power level stated in the manga. However we can make a few easy assumptions:

Lets take the first manga chapter where we see Ichigo easily defeat 4 skateboard guys. We also see a completely helpless ghost. I'm going to break it down to Ichigo, skateboard guys, and the ghost.

Lets assume that a baby would be power level 0.
Is the ghost stronger than a baby? Probably.

Using DBZ as a reference as seen here:
Is the girl (as a ghost) stronger than an old farmer with a gun? No.

We can assume that the ghost about level 2.
Ghost girl level = 2

Ok so each one of these skateboard guys is going to be stronger than a little girl ghost or no ghost.
Would one of these guys be, say, twice as strong as a little girl? Yes.
Are these skateboard guys as strong as an old farmer with a gun? I would say possibly. These guys aren't very brave so for the sake of the argument lets just say that these guys are around level 4.
Skateboard guy level (each one) = 4

Right so now lets look at Ichigo. He destroys the first guys easily. He could be 4 times as strong as one guy.
Considering that Ichigo is taking on 4 guys at once (whether they fight or not), he knows he is stronger than all 4 put together.
Ichigo is at least level 16.
Is Ichigo trying? No.
Ichigo is at least level 32.
Is Ichigo mad/serious? No.
Ichigo would probably be about level 45.
Ichigo (human form) level = 45

Now there is a short hollow/Rukia fight scene but we will skip it and go to when Rukia and Ichigo interact in Ichigo's room.

Rukia gets kicked by Ichigo when he's not serious and she isn't hurt at all. Rukia then easily binds Ichigo with a low Kido.
Rukia is at least level 16.
Is Rukia trying? No.
She is at least level 32.
Is she mad/serious? No.
Rukia is probably around level 64.
It is confirmed that Ichigo and Rukia are near the same level as you see them both get swatted by the hollow(pages 33 and 37) and they are hurt about the same amount. Ichigo could have been hurt more but the baseball bat absorbed some of the damage.
Is the hollow twice as strong as Rukia? Probably not since she's able to slice into it's arm.

Hollow is not level 128.
Since the hollow is able to damage them he has to be higher than their current state.
Hollow is at least 32.
Is the hollow trying? I would say kinda?
Hollow is level 38.
Is the hollow mad/serious? No.
Hollow level around 65.

Ichigo gains all of Rukia's power.
Ichigo would be max level 109.
Is Ichigo twice as strong as the hollow? No. It takes him 3 strikes to take the monster out.
However, was it hard for him to kill the hollow? Not really.
You will also notice that Rukia only intended on giving Ichigo half of her power. That would have made Ichigo's power level be about 77.
Rukia new that half of her power would give Ichigo enough to kill the hollow.
Now if we go back to the first hollow Rukia killed we get this:
Rukia max level is around 64. Is she twice as strong as the hollow? Possibly, but her first strike wasn't clean through.
Was Rukia trying when she killed the hollow? More than she wanted to show.
I would peg this first hollow around level 35.
So you add it all up and for the first chapter of the manga you get the start of the power level that looks something like this:

Girl Ghost - 2
Skateboarders - 4
Ichigo(Human) - 45
Hollow 1 -35
Rukia -64
Hollow 2 -65
Ichigo(Shinigami) -109

Now go to the other end of the scale and lets start with finale of the last arch.
Ichigo and Aizen could level mountains with the air from their attacks. Are we ever going to see people this powerful again? IF we do it wont be for a long time!

I think it would be safe to say that Ichigo was the pinnacle.
We will give him power level 1,000,000.
Aizen was stupid strong but it was clear that Ichigo was stronger.
We can give Aizen a number something like 990,000.

Fill in the blanks!