I'd have said Switzerland because of the sheer numbers of people in the country (in relation to the size of the population) who own firearms, the complete integration between military and civilian shooters even if weaker now than it once was, and the widespread use of service weapons - old and modern - in competition. The Swiss set up is so good we have a British society called the 'British Alpine Rifles' whose members spend some weeks there every year and shoot things like the Sig P210, K31, Stg57 and StG90 in competition with the natives. Outside of the major cities, some of whose inhabitants are anti-gun, shooters ride around on buses, trams and trains to competitions openly carrying their comp rifles including both their militia issued and privately owned models of the current Swiss service StG90 assault rifle without raising an eyebrow among other passengers. (This would cause mass public hysteria and panic in the UK these days with vehicles surrounded by MP-5 toting special armed police squads in no time flat.)
Britain does well with its access to long-range rifle competition shooting on both private and military ranges with 1,200 yards available on a couple of sites (Bisley and Blair Atholl) and 1,000 on several and a strong tradition of taking part in international competition in the fullbore disciplines both at home and through touring teams shooting around the world. The links to the former empire, now Commonwealth, countries - especially Canada, New Zealand and Australia, maybe South Africa too see a lot of competition shooting with our teams and people going there and theirs coming here. The still large (but not as vast as it once was) 'Imperial' at Bisley every year is the jewel in this crown with sling shooters entering from around the world. There are a lot of such service matches too where serving members of the armed forces shoot against each other both internally and internationally. The overt linkage between our forces and civilians that the Swiss have and the USA has with the Camp Perry and CMP matches with troops in uniform shooting alongside civilians has virtually gone here - we're (ie civvies) barred from entering their comps and while they can enter ours, can only do so as private competitors in their own time, out of uniform and with sporting club or private rifles of the same types are allowed to the public legally, ie manually operated.
Despite semi-auto rifles and all modern pistols being banned for civilian use we've retained and develop a lot of skills in quasi-service sniper etc comps too with several of our people doing well in international comps in Europe. We even have our 50-cal boys and a few girls at Raton every year for the 1,000 yard comps and the King of 2 Miles afterwards.
Germany is very big in club target shooting of all types - I'd think much bigger than us - and there are lots of collector-shooters of military weapons of all ages and types as well as a good number of F-Classers who travel to the UK for major matches with us, especially the annual F-Class European Championships which our Mik Maksimovic and his wife Tina have built up into a week long shooting festival held at Bisley every September with 200 + entries from all over Europe from Finland and Norway to Russia and Ukraine. (We see the occasional American at the F-Euros too - Biff Conlon over a few years back for those who know him.) Germany, like many European countries is both helped and hindered by its ranges for competition - helped in that there are many modern superb 300 metre ranges; hindered in that there is little available for longer distances except military facilities with the limited access to them becoming ever harder and rarer. One result of that is that many of our neighbours specialise in 300M 3-position shooting, CISM / ISSF and in Biathlon type events especially skiing Winter Biathlon, neither type big in the UK.