The current FirstGroup liveries as seen on FTPE, FCC, FGW etc rather than the original version seen on First North Western etc.
GNER
Transpennine Express in Northern Spirit days.
Any of the current SWT or EMT liveries
I also didn't mind the One Railway livery
Worst liveries..
Arriva Trains original - basically identical to the bus livery
Any liveries that are almost plain white like National Express
Great Western 'fag packet'
I recall WAGN had a strange livery at one point
And as has been said, any livery that is someone else's with stickers all over, especially when they are kept like that for many years - TPE after the First take over, the state of the East Coast mainline for the last few years, the current mess of liveries in the eastern counties.