The Netherlands, and then our region called “Twente” in specific. Why? Many shows to train and compete your horses. Close to the German border, many great showjumping stables in the neighbourhood like BWG stables, Jeroen Dubbeldam, Willem Greve and Gert-Jan Bruggink. The level of sport is high, courses are well designed which increases your own riding level as well.
Either Holland or the Dutch part of Belgium! The only reason I specify the part of Belgium is because I like the food better :) Holland and Belgium are super convenient in terms of transport when competing in Europe, which makes the trips less onerous on both my horses and grooms. Producing horses is also a lot less expensive in Europe, and there are a ton of shows at diverse levels nearby.