I don't stick to any one brand - I feed what is suitable for the individual horse or pony.
At present, I have three on Bailey's Everyday High Fibre Cubes, whilst the other is on Marriage's Horse & Pony Mix at the moment (because we won 7 bags of it!) plus sugar beet, but normally has oats rather than the mix when I actually have to pay for the feed!
In the past I have used Dengie Alfa A and Alfabeet as well.
I tend to use whatever has gone out of date or is ripped - we own a feedstore! So, at the moment they are on out of date Mollichaff Calmer chaff, Crediton Milling Equicubes (local mill), speedibeet (but once finished I'll probably go to A&P Fast Fibre), Spillers Equivite vit and min supplement, Equus winter glow, and NAF joint supplement... and out of date Equus spearmint!
Mixture, but at the moment just baileys lo cal balancer. I like badminton horse feeds and horsehage mollichaff. The local feed merchants own brand feed is quite good (and a lot cheaper
saracen and dengie mostly - saracen are only based down the road from me so can go and talk face to face with a nutritionist and I think it only them and baileys that do an endurance mix and haven't always had the best of times of baileys feeds
I use Simple Systems, I have a veteran, endurance horses and a yearling and find that these feeds suit them all and the advice from Jane Van Lennop is excellent.