i use a mix depending on the frame material, and what colour i want.
for good frames and simple colours, i tend to use Matt at the Powdershack, easiest way to find him is facebook, just put powder shack in the search bar, he did my modern aluminium O, my alpine 6, my fat bike, several for customers as well, he's cutting back a bit now though, working for individuals more and less for shops, shops kept asking for a "trade price" on his work which he found annoying, basically the shops were trying to make a mark up on his work, i charge the customer what he charges me, plus the p+p. the frame comes back ready to build. currently matt has a cannondale frame, also a swing arm and fork and my muddy fox frame in process
local alloy wheel poedercoater, 500 metres from my shop, ok for car wheels and gates, i tried him on an old frame, so much shot left in the frame, extra thick, all threads were a nightmare despite blanking etc, it was only a bike i built with a basic sunrace groupset as i wanted to compare it to basic shimano, essentially a testride bike for customers to put their minds at ease if i suggested sunrace as an option, but it was a nightmare to get the bb in, had to cut the seatpost down as the inside of the seat tube was a mess and i didn't have a reamer for a really old massive size so had to clean it as best as possible, so cheap powdercoat equals lots of time and hassle to rebuild the bike, never again!!
back in the day it was Argos and Bob Jackson for paint, obvs BJ closed down, Argos i've had some issues but they've also done great work, my Muddy Fox Courier comp was done by them, so was my Fat Chance Buckshaver, both great jobs, my issue was not understanding what i wanted as opposed to quality though.
i currently use Blue Whippet paintworks, which is Jason Rourke, i think the quality of the paint is great, that said they did put the wrong stickers on my muddy fox seeker, but i can live with that, they also did my Moser, my kilahonzo, my homage explosif, currently have an old raleigh up there for a respray for a customer.
i think the takeaway is find a good place and stick with them, that's what i have done, accept also that sometimes mistakes do happen, hopefully they can be sorted out, most are communication related, like with the stickers on my muddy fox.
examples of a few below
powder shack my aluminium O, picture does not do this justice:-
powdershack my alpine 6, white with a slight golden glitter in the laquer:-
my fat bike, powdershack pink!
blue whippet painted Moser, the camera can't cope with the flouro pink! :-
my kila honzo, by blue whippet
my fat chance by Argos