Not sure if you mean online ones or actual catalogue ones? Catalogue-wise, Ive never had a problem with Shires or Derby House. I havent bought from Robinsons in ages as they seem to do their own stuff nowadays which I dont really like
Online, Ive always found Equestrian Clearance very good with really quick postage.
depends what you want! Derby House used to be good but have been a let-down recently for me. hunting about online and then ringing to see if stuff's in stock is probably the best bet tbh.
If you're looking at on-line stuff I've always found both Rideway and Frogpool Manor have always been great for goods being in stock and quick delivery.
Horse and Rider Sheffield Very good - not the biggest range but good prices, very helpful, can get things in if they don't stock and all arrives promptly. Plus they do two P+P rates ie a small pkt rate and they clearly label all items as to what postage rate applies.
I've found Robinsons to be quite good but TBH I've found that returning things that don't fit or are faulty is a lot of hassle. I prefer saving like mad then having a big splurge at events like Your Horse Live where I can see the products and try them on etc, and they generally have reduced prices too.