“Best” is always a bit subjective. Assuming you are looking for what is considered to be a “normal” landscape lens (Wide/UW-Norm zoom) you are looking for something in the 12-24, 15-35mm range. Price of course varies and “expensive” is also subjective.
The RF 25-105mm is a great lens And will probably handle majority of Landscape shots, assuming you are looking for something with more ultra-wide range, I’ve had the following lenses that all qualify as top rate Landscape lenses over the years. They all have pros and cons. I see you are in the UK while I am in the US so prices I note below are USD and likely higher in UK.
EF 17-40mm f/4L - Long time workhorse in the Canon lineup. Getting a little long in the tooth, but still gets good reviews. Cheapest new at $799 than other options, due to the number of them out there can find good copies much cheaper used. Starting at 17mm it doesn’t get as wide as the others. I had one many years ago, but didn’t keep it very long As the range just didn’t work for me.
EF 16-35mm f/4L - Great image quality, and at $1099, cheaper than an f/2.8, but only f/4. Not a problem for landscape where you will usually use smaller aperture, but for indoor use the extra stop might be important, bit not too much of a concern given the low light sensitivity of the R. I purchased this lens when I started doing more and more real estate work. Ended up selling it, but only after I made enough money with it to buy the EF 11-24mm.
EF 11-24mm f/4L - Also, great image quality and SUPER wide, but also SUPER expensive ($2999 new). Also only f/4. Very heavy lens and for some landscape shots 24mm may be a little too wide. I used it a lot for real estate work as the 11mm with no distortion is great inside for tight space. I took a pretty serious hit when I sold it, but that was softened somewhat by it also being a lens I made good money with. Sold it to get the RF 15-35mm when I decided to go all in on the R mount.
RF 15-35mm f/2.8L - Native RF mount, fast 2.8 max aperture. Pretty compact and light weight considering build quality. $2299 means it isn’t cheap, but considerably cheaper than the EF 11-24mm. After serious thought, I realized the f/2.8 and 24-35mm range was more useful to me than the 11-15mm range on the EF 11-24mm. To make up for the loss of the super wide, I did however pick up a 12mm Laowa lens for tight spots inside. 12mm Laowa in RF and the RF 15-35mm combo gives almost identical wide end, longer long end, f/2.8 and combined can be had for less than the prices of the EF 11-24mm and no need for an adapter.
Other lenses (I have no personal experience with)...
Tamron 15-30mm f/2.8L G2 - I nearly bought this lens twice. It gets fantastic reviews and and mixes a fast 2.8 aperture with bombproof construction, great reviews for image quality for a very reasonable $1299.
Finally, there are rumors of an RF 10-24mm f/4L in the works. I imagine it will be priced similar or more to the EF 11-24mm Also rumored is a Canon RF 14-35mm f/4L IS USM. Of these two - I would expect the 14-35mm to be a great balance of features and price similar to the EF 16-35mm f/4L.
