Special mention for
Condor cycles from whom I've received great customer service recently (as has always been my experience).
SJS are always great to deal with and carry a lot of gear but are rarely really competitive on price.
The last load of Shimano stuff I got came from Bikeparts and Bike Tart; the latter typically the cheapest I could find a lot of stuff.. the former sometimes close but sometimes subject to weird pricing fluctuations and I did fall foul of a stock cock-up a while ago which was put right but still a pain.
Also had good experiences with (IIRC) Active BPM on
ebay, although I try not to buy through the platform.
Hopkinsons cycles are great for the odd random cheap item (and are my go-to for KMC chains), Bankrupt bike parts are also good if you're happy to sift through an eclectic mix to find the right bits. Also had some decent cheap kit form Paul Milne cycles.
Also had good experiences with Fawkes cycles in the past and Tweeks.
For more left-field stuff Bikemonger are good and Fresh Tripe have a good rep although IIRC I've not had anything from them.
I've also had a few well-known Chinese-origin bits from various sellers on Ali Express with no issues.