I think the liveries should follow the features of the body of the vehicle. That would mean mainly straight lines. I don't like swoops and bumps that conflict with the vehicle's own body lines, and odd blobs of various shapes stuck onto the side with no evident logic.
Silverlink and EMT liveries include unnecessary bumps on the lower sides. Most Northern liveries look a mess, though the one on the 319s isn't so bad because the wavy lines have been kept under control.
If you have it available, there's a picture of VTEC's new livery on page 8 of the April issue of Modern Railways. I spent some time wondering about the large slanting coloured bit on the first Mark 4 coach, till I realised it's actually the shadow cast by the adjacent footbridge.
I think the WAGN livery on post 4 looks good. The curved line is controlled and the big contrast between the colours gives it a kind of strength.
If I was running a TOC, I would not be a customer of Ray Stenning's company.