Depends entirely on the job, mate. Trade isn't one market — there's volume contract work and there's high-end private work, and the brand that's right for one is wrong for the other.
For contract and volume work, Dulux and Crown are the backbone of the trade for good reason. Their trade lines are formulated for speed — good coverage, sensible drying times, easy touch-ins, and you can get them at any decorator's merchant on the way to site. Dulux Trade Vinyl Matt and Crown Trade Clean Extreme are the kind of thing you'll see on a hundred jobs a year. No drama, no surprises, easy to colour-match for the client.
For premium and private work, this is where the supported brands earn their money. Benjamin Moore is the decorator's secret weapon — their Regal Select and Aura lines lay off beautifully, touch in like nothing else, and the colour depth is superb. Self-priming on most jobs too, which saves you a coat. Little Greene Intelligent Matt is genuinely wipeable and the colours photograph brilliantly for your portfolio — Intelligent Eggshell on woodwork is a cracking trim option.
Mylands is worth knowing for high-spec London work — proper depth, made in-house, and the trade understands it as a step up.
Now the "but what about Farrow & Ball" question. It's a beautiful paint and clients love the name, but the older Estate Emulsion was fussy and slow on big jobs. Their newer Modern Emulsion is far more forgiving and wipeable — that's the one to spec if a client insists on F&B for a busy room. Charge accordingly for the slower work.
Practical advice: keep Zinsser BIN and Cover Stain in the van regardless of topcoat brand — they sort stains, knots and dodgy substrates that no emulsion will fix on its own. Match the topcoat to the client's budget and the room's wear, not to loyalty.