Failing that, DRS is usually good, although I quite like Freightliner. Going back, LoadHaul was the best early privatisation loco livery (excluding GNER), with Intercity Swallow for BR days. Rail Blue only worked in Large Logo form, which cancelled out the drabness, although it looked good when clean.
Technically, Load Haul was pre-privatisation, being one of the "shadow franchises", or in this case, FOC, set up by British Rail.
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My favourite current livery is in use by Riviera Trains and most of the coaching stock at Didcot Railway Centre
If you want me to be serious, then I'd have to agree with sprinterguy about GNER, GW "Merlin", original MML, Saltire (which looks remarkably like Glasgow Blue Train), Chiltern and Valley Lines. I quite like Southern's slam-door scheme as well, and East Coast's "Silver Jubilee" livery is good. What I don't like is the massive over-use of white (as in Southeastern, Southern modern stock, WAGN original and NXEC). Paint something white, it inevitably goes a yucky grey or yellow before long. Eurostar got away with it, mostly by cleaning their trains a lot. Those horrible "swooshes" used by First and Stagecoach are bad too. They sort-of work on stock with raked-back ends, but not on units with corridors.
Going back a bit, NSE was nice and cheerful, RR was fairly good and Intercity Swallow had a good, businesslike look (if a bit dull). Rail Blue only ever looked good when clean, otherwise it was just drab. Maroon was similar, while crimson & cream was much brighter. Chocolate & cream is excellent, crimson lake was a much nicer shade than maroon, both shades of Southern green were nice, but the best livery ever was the Coronation colour scheme. Garter blue and Marlborough blue, worked perfectly.
<EDIT> How could I forget GC Zephyr (when applied to 180s)?
I have to say, we seem to be completely in sync when it comes to livery preferences! I agree wholeheartedly, and am ashamed that I too initially forgot about the Grand Central "Zephyr" livery, which IMO is the first decent livery to grace a 180 and show off its' lines to full effect. I was thoroughly non-plussed with GCs' original all black colour scheme (if you can call it that!) though.
Similar to you, I loath the bus company liveries that have been applied to TOCs: Stagecoach "swirl" on EMT and SWT, and First Group "barbie" livery, as well as National Express "Connecting Lines". Trains should not be thoughtlessly daubed in the same colours that the owning company paints their buses in. Arriva (Wales and the old ATN franchise) also fall into this category, but I actually think that the new ATW Executive livery is rather nice, as there's actually been a little bit of thought gone into it as to how it would suit the trains it is being applied to.
I can't stand Dynamic Lines either, it's an absolutely shocking livery treatment for a rail vehicle and fails to convey any sense of impact while being unnecessarily messy. Just because certain fancy effects are now possible with enhanced vinyl technology (and perhaps this isn't the thread to be saying this!) doesn't mean that they should be used.
Liveries with large unrelieved expanses of white are a definite no-go for me: They completely fail to inspire and become dirty very quickly: EMT, SWT, National Express corporate, the old Connex liveries and the new Greater Anglia scheme all fall into this category. I don't mind Southerns' livery though: The contrasting shades of green in their "roundel" arrangement are very pleasing.
The current fashion for liveries with large areas of unrelieved grey or silver don't do anything for me either: The purple stripe on East Coast is placed too high up and is the first livery to be applied to them that has made the 225s look anything other than sleek and "in trim" (Intercity swallow was best for beneficially accentuating their sharp lines). London Midland is a bit bland for similar reasons, and Virgins' current scheme doesn't do much for me either. However, I am keen on the current Crosscountry livery: I think it is the use of darker colours, and the dramatix "X" symbol that helps to offset the otherwise uninspiring effect of the large expanses of silver-grey.
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In general, the liveries that I really
like are those that use fairly simple bands of colour and strong contrasts to get their brand across. This would clearly include the current Northern livery (which reminds me somewhat of what an updated Provincial scheme could have looked like), Northern Spirit, Great Western Trains, GNER and Anglia Railways.
I also like liveries that have been designed to be mindful of some sort of heritage related to the operation, or to the geographical area served: So Northern Spirit with the big "N", the "Great North Eastern Railway" title and "Route of the Flying Scotsman" tag, the current Scotrail livery of blue with the white Saltire, the old National Express Scotrail "swoosh" livery that adpated the traditional Scotrail "swoosh" logo that was introduced after sectorisation in BR days, Great Western Trains aiming to make use of a dark green principal body colour and Southern adopting a two-tone green livery. And Valley Lines made use of the colours of the Welsh flag of course and IMO would make a good basis for an all-Wales livery.