Obviously the 4 slams are the most important tournaments, but after that, which one of these three huge events would you like to win the most? Saw a couple debates about this lately, so why not make a thread about it
Davis Cup has the honour of fighting for your country, and it still has best of 5 format. Is more of a team sport however, so you could profit from other gifted players (or be screwed if you're the only decent player from your country, like Murray).
Olympics is the biggest sports event in the world, has the history, a place where legends are made. Everyone wants to be an olympic gold medalist. Tennis hasn't been there for very long though, and therefore it lacks the great champions/stories from the past. Still has best of 5 in the final.
To win the WTF you have to beat 4 or 5 top 8 players, arguably even more difficult to win than a Grand slam. But, a lot of the top players are tired after a long, hard season, and come here in terrible shape. And now it's in best of 3 format, and we all know the truly great matches are fought out over 5 sets.
I'd say Olympics >> Davis Cup > WTF