OK, my experience.
As a whole, the most polite people I have ever met. Very nice and helpful. Even if you don't ask for help, they will come to you and ask if you need help.
Everyone in Serbia was very nice and welcoming, but I'll never forget this one guy I met on the train to Belgrade. He was Serbian and travelling from Austria, where he lived and worker, to Novi Sad in Serbia where his family lived. We were sharing the same cabin (him, me and 3 friends of mine) and we were asking him about Serbia, what to visit in Belgrade, etc.. He was teaching me some Serbian sentences to shout at Nole (I was travelling to watch him play DC). Then we fell asleep and when we woke up he was gone because his stop was before Belgrade. he had left us a box of chocolates and a farewell message.
Bosnia and Herzegovina:
Ironically, my experience in Bosnia was very similar to my experience in Serbia, people were very nice and helpful in the same way.
People were just generally nice, had a pretty awesome experience at this traditional restaurant.
Generally, people were not nice. This lady at this souvenir shop was acting like she was making me a favour.
I really liked the places I visited, but not the people.
Having lived a year in Budapest, and loved the whole experience, the city and the country, I must say that Hungarians are not welcoming at all. They are rude, the language is as difficult as it gets, they know you don't understand it and they don't care, they'll still talk to you in Hungarian. I warmed up to the people during my stay, but that's because I stayed a long time and got to know them better. For the regular tourist staying only a few days, they are not welcoming at all.
Most welcoming : Ibiza - Went when I was 13, so obviously no clubbing element really available for me at that age, still got a great Holiday out of it, atmosphere was great, locals were friendly, drunk tourists were friendly, reps were friendly. Never felt uncomfortable.
Well the whole place is pretty much designed for tourists. Having been there, I think it's pretty hard to assess whether the locals are welcoming or not, because you pretty much have close to zero contact with them, the whole Island is tourists and people working in tourism (and most of them aren't even from the Island)
That is if you go there in the summer months.
Spain - Mainly Spanish speaking area, some may of found it difficult to survive. Was with School and locals again very friendly, despite language barrier, no strange looks because we were foreign or any thing like that. Great hosts.
I think that the language barrier shouldn't be a factor in the equation for the most welcoming country. For example my own country - Portugal -, the older generations do not speak English, but I believe that is part of why they seem so nice and welcoming, because despite the language barrier, they make the biggest effort to help you and make you feel comfortable.
Least welcoming: France - Always enjoy my time in France when I go with the family. My aunty has a house there and we go there quite often, used to also stop off at Eurocamp parks for a week before heading on to my aunties. Find the French natives rude and arrogant, not very helpful, and look at you as if you are aliens. They always push in queues for things and the drivers there a dangerous as fuck. Only really welcoming French I have encountered there is those who worked as Reps in the eurocamp parks, restaurant workers, or the locals that live in the houses by my Aunty's place.
From my experience, the French can be really though but also really nice. It helps if you speak the language, or if you make the effort to learn it or speak the basics.