The giro is usually much more exciting and fun to watch then the tour.
That depends on how much we get to watch.
Since Lance's retirement, the Giro has seen a reduction in coverage on Versus. I believe last year you only got an hour wrap-up type show each weekend. Same for the Vuelta.
The Giro (Italian Tour) and the Vuelta (Spanish tour) are much more national races. Although the Tour de France is the same way, it has become much more international simply because of the hype that international riders put on it.
The organizer of each tour is required to invite the core group of teams (assuming they are free of any scandals). Then, they have 3-4 wild-card invitations. The wild-card invites are done in the months leading up to the event. The organizer will favor the teams of the nation hosting the event.
Basically, the Giro has many more Italians, Tour has more French, and the Vuelta has more Spaniards. If you look at the ToC line-up, you'll see a bunch of American teams that you don't see racing in Europe. The teams of the home country lobby hard to get in to expose their sponsors to the audiences
The Giro is the first
Grande Tour of the year. That increases the urgency. The Giro and Vuelta tend to have shorter stages. The riders fatigue less and have more punch the next day. The Vuelta is not until August-Sept. Many riders are burned out and just going through the motions. Others are trying to make up for a poor performance earlier. If you are into Spaniards, it is cool. Otherwise the Vuelta is the least entertaining.
The TdF always draws the biggest stars. The media treats it like the Superbowl. A winner of a single TdF stage will eat and drink for free in France and his home country for the rest of his life. The stages are longer. Many racers and teams actually try for the other awards offered. There are four jerseys but also team competitions, etc. Although the race for Yellow may get boring, the many races within the event can be very entertaining. The more you pay attention, the more entertaining it becomes.