The one you really want to go to.
Think about what 'Europe' is for you.
For some people it is London and Paris, for others it is Norway and Ireland, or just the Alps.
If you feel that way about Europe, only a visit to that place will do for you.
If you do not have that 'this is Europe' view, you might consider going to a country you have links to, like ancestors who came from there, or friends.
If you are really open to all of Europe, you can go by your budget and the climate or weather when you want to travel.
In March or April, the Mediteranean area is nice and mild, while Norway and Scotland are still cold. But in August, unless you love hot weather, you will prefer Scotland and Norway.
Or look at your budget and find a part of Europe to fit it.
London and Paris are in the $200 per day range, for backpackers.
While in rural Poland you can get by with about $75 for the same kind of comfort.
There is no country in Europe where traveling is hard or impossible, most countries in Europe are easy to travel to and in.