Circular polariser
Discussion
Personally I use a Hoya HD circular polarising filter (£95 for the 77mm size).
It's about twice the price of the Hoya Pro1 polariser but the HD version is slimmer so less issues with vignetting on wide angle lenses, and it has very good light transmission (they claim 25% more light than most others). In fact, I tend to leave it on my Canon 24-105 f4L lens most of the time, unless shooting in low light. In the past, with other polarisers (cheaper Hoya and before that a Cokin one) which blocked more of the light, I'd always remove them as the light loss was significant.
I haven't any experience of more expensive ones (such as Lee) and they may well be superior, but I have no issues with the Hoya HD, and have been using it for just over 10 years now (bought it in July 2009).
Both the following were taken (recently - in October 2019) with the filter attached, the second with it oriented to cut out the reflections from the water, and improve the sky a little. Click through to FlickR for the high res view to check on sharpness etc..
3O2A1255 by conradsphotos, on Flickr
3O2A1256 by conradsphotos, on Flickr
It's about twice the price of the Hoya Pro1 polariser but the HD version is slimmer so less issues with vignetting on wide angle lenses, and it has very good light transmission (they claim 25% more light than most others). In fact, I tend to leave it on my Canon 24-105 f4L lens most of the time, unless shooting in low light. In the past, with other polarisers (cheaper Hoya and before that a Cokin one) which blocked more of the light, I'd always remove them as the light loss was significant.
I haven't any experience of more expensive ones (such as Lee) and they may well be superior, but I have no issues with the Hoya HD, and have been using it for just over 10 years now (bought it in July 2009).
Both the following were taken (recently - in October 2019) with the filter attached, the second with it oriented to cut out the reflections from the water, and improve the sky a little. Click through to FlickR for the high res view to check on sharpness etc..
3O2A1255 by conradsphotos, on Flickr3O2A1256 by conradsphotos, on FlickrEdited by C&C on Saturday 4th January 12:31
After a lot of testing, I settled down with Marumi CPL. Actually, I've got three of them, one on each lens i use for car photography 
I saw somewhere a couple of tests and Marumi gave slightly better results comparing to Hoya (unless they updated their filter and my knowledge about it doesn't matter anymore)
Not only this is practically the best CPL filter, but it is also slim. It really matters when you put on a wide-angle lens and maybe you want to put another filter (for example ND + CPL).
I saw somewhere a couple of tests and Marumi gave slightly better results comparing to Hoya (unless they updated their filter and my knowledge about it doesn't matter anymore)
Not only this is practically the best CPL filter, but it is also slim. It really matters when you put on a wide-angle lens and maybe you want to put another filter (for example ND + CPL).
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