It has always been the case that a fundamentally good livery can be ruined by minor variations. First's Dynamic Lines livery looks incredible on the 170's, 185's, 165/6's and especially the 180's (hard pressed to find a better looking train and livery combination), but looks awful on the first capital connect fleet and the rest of the FGW fleet.
Grand central livery is very nice indeed. The removal of the orange stripe on the 180's would make them perfect.
The final version of the Northern livery is very stylish and looks good on whatever unit they are applied to (actually making pacers look rather desirable externally), though earlier versions of the livery left a lot to be desired (White and purple 156's anybody?). I would also not be too disappointed to lose the red/purple liveries from the Yorkshire EMU's.
I very much like London Midland livery - very smart - though it does not suit the Class 350 very well, again, probably due to the slight variation.
Cross Country livery has grown on me, but still looks better on the turbostars than the voyagers. It certainly does not suit the HST.
I personally was a massive fan of railfreight triple grey livery - a livery that has not been bettered on freight since.