I started with Ernie Ball 9s. Then 10s.
But I really didn't get on with them, so I switched to Dean Markleys (10s)
I used them for many years and then started to break them a lot in the late 80s when I was gigging more heavily (and my strat's bridge was wearing out!). The luthier's suppliers that I lived next door to at the time (Touchstone in Reigate) talked me into switching to the main brand they stocked. I think they were branded as Schallers, they were very nice. I used buy boxes of them rather than single sets, so that I always had strings in the gigbox (and they did me a deal :D).
Then about 10 years ago I needed strings in a hurry and bought Rotosound (the very set that arrives with BKPs!). But when I switched to Gibson scale-length I found I had to move up to 11s, RotoReds(11-48). After that I found I preferred 11s on Fenders as well.
I was very happy with those until I got a new strat about a year ago. I wanted 11s to put on it, the shop were out of RotoReds, but the guy said "try these, they're loads better" and threw two sets of D'Addario XLs (11-49) in the case.
I only buy RotoReds if I can't get D'Addario now (and the extra top Es are still handy!).
For some reason, if a humbucking guitar needs stringing they get RotoReds if there are any left. The Strat and Tele only ever get D'Addarios.