can someone tell me the best navcore for go910

The best is always what Home offers - in your case the 7,903.
I think you are more looking for something that can work on device but is of a higher version number ;)
The X10 devices can be a bit stubborn when it comes to a new Navcore so you might habe to test a few until you are satisfied.
A good start would be the 9.004 or the 9.205 or for the V8 Navcores the 8.562 or the 8.417.
As an alternative you can use the DSA tool to download, patch and install any Navcore you like - but same as above, you might have to try a few.
If you have the HDD replaced by a CF card I recommend the DSA tool as it has a special patch for the CF cards included.

B/L 5.5229, NC 9.026 work fine on my GO910 with W&CE 880.3810.

Thanks for that info!
Can you also confirm that there is no problem with the USB transfer? Like drop outs or extreme low transfer speed.