Is there anything resembling a consensus on Lloyd's most successful symphony, i.e. a place to start?
Sounds like everyone else is pointing at 4 and 7. If I may offer a different point of view - I don't think I've ever heard 7, so at a bit of a disadvantage here. My intro to Lloyd was the Lyrita 3CD set of 4, 5, and 8, and I immediately hit it off with 5. It's something I listen to about twice a year for over a decade now. It's quite hard to describe because any description of it sounds stupid (part pastoral romp, part tragedy, with the full orchestra only playing together in the second half), but the parts all work together cumulatively so very well. 4 is also good.
But I would start someone off with 6. The reason is simple: it's the short one

It's only like 20 minutes, and you get the full Lloyd experience, including a romantic time warp slow movement. If you like 6, then proceed on to 4 and 5, which are an hour each, and consider some of the other stuff, which is also ... well, usually also an hour each.
This answer may depend on how you plan to sample. On CD, No. 6 comes with a fun little overture and another symphony, 10, which is for wind band and not a top recommendation of anyone's. So No. 6 may be a superior starting point if you are streaming or trying it out on YouTube, but if you want to start with paying for a physical CD, maybe you should after all go with 4 and/or 5. As for me, guess it's time I tried No. 7...