As a Liverpool fan definately the league. It's too painfull that i haven't seen us lift a league title in my lifetime 
Someone said above that if you win the league then it proves your the best team in England, i don't neccesarily agree with that. ATM i'd say Manchester United are the best team in England :'( because they've won it 3 years on the boucne. You can't really say a team's the best in England if they win it one season then don't win it for another five. However i'd say the best team in Europe is the Barcelona (obviously) but this is because they won the league, the copa del rey, and then defeated United (the best team in the best league in the world?) in the champions league final. thus meaning there better than everyone else.
So to prove your the best in your country, win the league consistently.
To prove your the best in the world. Add a champions league trophy to the consistent league trophies.

Someone said above that if you win the league then it proves your the best team in England, i don't neccesarily agree with that. ATM i'd say Manchester United are the best team in England :'( because they've won it 3 years on the boucne. You can't really say a team's the best in England if they win it one season then don't win it for another five. However i'd say the best team in Europe is the Barcelona (obviously) but this is because they won the league, the copa del rey, and then defeated United (the best team in the best league in the world?) in the champions league final. thus meaning there better than everyone else.
So to prove your the best in your country, win the league consistently.
To prove your the best in the world. Add a champions league trophy to the consistent league trophies.