Soccer of course. But since Europe is full of strong clubs and countries in soccer, it's very hard to reach good results.
Anyway Hungary is very strong in sports. If we would look at the population and gold medal ratio only, in Olympics, we would win it all the time.
Water-polo. swimming, kayak-canoe, fencing, handball are also popular sports.
In Australia these are:
1 - Swimming (yes, exactly! With a MASSIVE 3.1 million participants, swimming is the most popular sport in Australia)
2 - Cycling
3 - Soccer
We have 4. Football, Baseball, Hockey and Basketball. I'm a big NASCAR fan. We had a truck race once in Denver the 1st season they started the trucks but nothing in my region since.
I am from Croatia. By far the most popular sport in my country is European football. That is for sure.
If you ask few people from Croatia which sport in their opinion holds the second and third place in terms of popularity in the country, you would probably get different answers.
In my opinion that can be handball, tennis or basketball.