Harry Potter and Transformers 2 are imo the likeliest to dominate the summer box office.
I think Wolverine will do well despite whatever negative feelings people may have towards X3. Just judging the box office numbers of the X-Men trilogy so far and it's fair to say that X-Men is are popular amongst the general audience which is why it was a good move by Fox to add
X-Men Origins to the title.
Wolverine and Hugh Jackman's popularity plus the inclusion of fan faves such as Gambit and Deadpool is sure to draw in the masses. Plus May has always proven to be a lucrative month for Marvel.
Gi Joe is such a tough one to call. It has a big fanbase through the animated show and merchandise but the promotion so far has been awfully lowkey for what's hoped to be the first in a series of films.