TBH it depends on the fit and mine have all been a bit uncomfortable initially until they have dropped a bit but I just stick heel lifts until they do. I guess having made to measure is the most comfortable option but can be a bit pricey. I'd go somewhere where you can try lots of different makes (big horsey events like Badminton, HOYs etc) and see what fits you best or shell out for made to measure.
Depends on your budget and style. I personally love my De Niros. They've been a little bit abused and still look fabulous. They've also barely dropped which is good for me as I really hate when boots drop too much and end up looking a little too short for you
I can't have the really stiff leather some of them are made of as I broke my ankle last year and they just hurt, I bought some mountain horse venice field boots for showing and they are so comfy and soft and not too expensive either.