Coverage on hilltops is very hit or miss, as you will normally be relying on accidental coverage from cells intended to cover some nearby centre of population. Sometimes the phone will be confused by receiving a strong signal from a cell site which is too far away to meet the timing constraints for 2-way communication.
I don’t think there is a single “best”.
If you are buying a SIM for the purpose, consider a “global” SIM. There are several providers of these, and they will usually allow you to roam onto any of the UK networks (except Three, which is a bit different). As a bonus, you’ll get lots of other countries too. The British companies offering such SIMs will typically issue a number based in the Channel Islands or the Isle of Man, which run their telecomms independently of the UK.
My own solution is that my main everyday phone is on Three, and I have a spare old phone with a global SIM for when that doesn’t work. In this way I can take advantage of whatever coverage there is. I only use the global SIM for SMS spotting since I don’t have a data bundle on it, so the cost of this extra SIM is negligible. I find I only have to “top up” every 2 or 3 years. You can of course buy data bundles if that is what you want.
When I look at which carrier my global SIM actually picks up on a summit, none in particular seems to dominate, though I haven’t done a systematic survey. I’m pretty sure that O2, Vodafone and EE have all come to the rescue on at least one occasion.
As ultimate backup I have a DeLorme inReach which offers messaging via the Iridium satellite system and will work anywhere that you can find open sky, but that’s another story.