[moderate]

Consider 3 oxidizing agents:

Pyridinium chlorochromate (C5H6NCrO3Cl)
Sodium dichromate (Na2Cr2O7)
Chromium trioxide (CrO3)

They are listed in increasing order of oxidizing strength.
Oxidizing strength is defined as the compound's ability to oxidize an alcohol. Stronger oxidizing agents oxidize primary alcohols to carboxylic acids, whereas weaker ones oxidize primary alcohols to aldehydes.

I've been thinking about this for a couple of hours but can't come up with anything satisfactory. I also haven't been able to find anything online/in textbooks.

I have developed a theory for the case when the same central metal atom exists in the compared oxidizing agents.

The more bonds connect the central metal atom to electronegative atoms, the more will the metal atom's electrons be withdrawn by induction, and the more electropositive (and therefore better at being an oxidizing agent) the central atom will be.
If an electronegative atom that is bonded to the central metal atom has a negative charge or is bonded to another electropositive atom, then the inductive withdrawal of electrons will be lessened, making the metal atom a poorer oxidizing agent.

CrO3 (strongest oxidizing agent)
http://chemicalland21.com/industrialchem/inorganic/1333-82-0.gif

It has 6 bonds to oxygens, none of which are negatively charged or bonded to electropositive atoms.

K2Cr2O7
http://www.chemyq.com/xz/img/img7/7778-50-9.gif

6 bonds to oxygens, but one of the oxygens has a negative charge (so it will not withdraw electrons as much from the chromium)

PCC (weakest oxidizing agent)
http://upload.wikimedia.org/wikipedia/commons/9/91/Pyridinium_chlorochromate.PNG

5 bonds to oxygens, and one of the oxygens has a negative charge (so it will not withdraw electrons as much from the chromium)
1 bond to a chlorine, which is less electronegative than oxygen, so it will not withdraw electrons from the chromium as much as the 6th oxygen in potassium dichromate

[Jorriss]

I haven't thought about this before but I'd probably look in the direction of the oxidation state of the oxidizing agent and what oxiation states the metal is comfortable with.

[moderate]

the oxidation state of the oxidizing agent and what oxiation states the metal is comfortable with.

So do you mean something along the lines of:

going from M⁵⁺ to M³⁺ has a  :delta: G of -2 kJ/mol whereas
going from L⁵⁺ to L³⁺ has a  :delta: G of -5 kJ/mol,

so, L will be a better oxidizing agent?

[Jorriss]

the oxidation state of the oxidizing agent and what oxiation states the metal is comfortable with.

So do you mean something along the lines of:

going from M⁵⁺ to M³⁺ has a  :delta: G of -2 kJ/mol whereas
going from L⁵⁺ to L³⁺ has a  :delta: G of -5 kJ/mol,

so, L will be a better oxidizing agent?

Not really, but maybe. It's not something I've thought of so I don't have the answer in my head =/. Just throwin' something out there.

[bromidewind]

Read up on how to determine oxidation states.

CrO₃ has an oxidation state of 0, due to the chromium's oxidation state of +6 (to counter the -6 from the O₃)
Na₂Cr₂O₇ has an oxidation state of +4
PCC is more of a doozy... I don't have the number off the top of my head nor do I have time to try and figure it out. But I would assume that it's somewhere past +4.

So the lower the oxidation state, the better it is at oxidizing.

I probably just butchered all that.. but that's the best of what I remember from Gen Chem and the infamous oxidation numbers lecture (what? you mean oxidation numbers are DIFFERENT from ion numbers?)

[uvcyclotron]

I have developed a theory for the case when the same central metal atom exists in the compared oxidizing agents.

The more bonds connect the central metal atom to electronegative atoms, the more will the metal atom's electrons be withdrawn by induction, and the more electropositive (and therefore better at being an oxidizing agent) the central atom will be.
If an electronegative atom that is bonded to the central metal atom has a negative charge or is bonded to another electropositive atom, then the inductive withdrawal of electrons will be lessened, making the metal atom a poorer oxidizing agent.

I find this theory good-enough. and it applies in this case too, but for comparison b/w different central atoms, i guess the  comparison of :delta: G for ion reduction (thermodynamic factor) would suffice, as the kinetic factor doesn't seem to play much role here. Though, I could be easily wrong, but this is what I think.

[orgopete]

I am not sure where this is going, but this is how I envision these similar reagents. Chromium trioxide is the anhydride of chromic acid. Chromic anhydride is the anhydride of two moles of chromic acid. Dichromate is the salt of that anhydride. I presume that addition of water can (but not necessarily does) hydrolyze the anhydride back to chromic acid. Chlorochromate is similar to chromic acid, but instead of adding water, HCl is added. Then it is converted to its (partially) soluble pyridinium salt.

Because sulfur exists in similar oxidation states to chromium, I would be reluctant to rely too much on the oxidation states.