i'm sure everyone will have a different answer....but here are my thoughts.

SFO base.
*To/from SFO, I far prefer the crew on CX873/872, simply because I've flown those flights so many times I know half if not most the FAs on each flight these days. I know a few well enough to give them a heads up I'm flying on (x) date. I've read other people here with mixed reviews of them, including my partner who complained about service in J last month on CX873. But selfishly, because there is a virtually nil chance I don't know half the FAs, my service is exemplary.

As an fyi, the other two SFO flights are HKG base, although you can get an SFO FA here and there due to irregular scheduling, standby, etc.

JFK base.
*They are regularly rostered on CX830 and CX845. Except in extraordinary circumstances they don't do the other flights. I've now had multiple flights in F on CX830 and CX845 since the JFK crew base was launched, and I am not impressed in comparison to my many other anecdotes on CX. Small things like forgetting the Chinese side dishes, serving me a dirty table cloth upside down (they put the clean side facing up, while the other side was soiled, either mine or or another pax from earlier in the flight!...and when I pointed it out, a shrug and "I'll get another one"), forgetting dishes ordered in advance before going to bed and then running out, generally seeming overwhelmed, and so forth.

It probably doesn't help that, with one exception 5/6, the last few CX830/845 I've flown have been a full house 6/6 in F, which always presents a challenge for the FAs except for the best crews IME. To be fair, it's very difficult IMO to have a "bad" flight in F on CX when you have 4/6 or fewer. I'm seeing a concurrent thread about JAL and will just chime in that when CX has just a few pax, it's hard to beat CX F IMO despite the less-robust catering and unbalanced drink list. This is a personal style thing but I really like the CX approach to service when it's on the ball (ring the light, not robotic, relaxed but prompt). But anyway, 6/6 just is one too many people in my opinion given just 2 FAs and only 1 is on duty for ~2/3 the flight, so that could be coloring my experience with the JFK base in F. Perhaps YMMV.

And I'll add, in J class I don't seem to notice a difference.

LAX base.
*They are rostered on CX882 and one of the LAX night flights. I don't have a regular LAX pattern - I pretty liberally use all four daily flights - and as a result I don't really have many thoughts here. Occasionally I recognize a LAX crewmember here or there, but overall they seem fine to me. I don't have a preference either way. I've had my worst few flights on CX with a HKG based crew to LAX. I know years ago there were some senior ISMs who swapped almost exclusively for LAX, and were notorious internally for being terrible. I had the luxury of experiencing their services a few times and concur. CX actually made an effort to clean out some of these "famous" ISMs, and succeeded in forcing retirement and in at least 2 cases I know about, finding cause to fire them. I haven't had those types of ISMs in a while.

I don't fly LHR or YVR enough to have an opinion about those bases.