Best upgrade for cd player?

I don't think there is one for nuance, detail and dynamics.

All CD player sounds are notoriously close and differences are subtle to say the least. Try comparing a budget machine to a top of the range one.

Or even a Blu-ray / DVD one.

On the other hand a careful loudspeaker upgrade might well give you all 3.

I have an Oppo 205.  A modest improvement can be had by (1) putting it on sorbothane feet, and (2) opening it up and putting dynamat on bare metal, and especially on the transport.  
There are a lot of aftermarket power supplies.  They seem easy to install.  I am skeptical but cannot say as I have not tried one. 
Don't go with snake-oil cables.  However if your pre-amp has balanced inputs then using XLR cables will give a very small improvement. 

My personal experience with sorbothane is that it sucks the life out of music. Check out Machina Dynamica springs, which I have under everything but my speakers, which are on Harbor Freight dollys

There’s a lot more to balanced then just having XLR I/Os. The circuits must also be balanced= more money=better sound

hth

Thanks to speakermaster and dinov. Helpful!

Get a good DAC.  That’ll give you more of an improvement than pumping more $ into an Oppo that’s out of business

+1 get a good new school R2R discrete dac with volume, and go direct to poweramp/s

Cheers George