Of the three SPG properties in Zagreb, I think the Sheraton is the nicest/newest. The Westin was built in the 70s or 80s (high rise concrete) as the Intercontinental and then became the Opera (independant) and then the Westin. The Sheraton was built in the 90s (as the Sheraton--lots of glass) and is closer to downtown than the Westin (although not by much)--maybe a ten minute walk to the main square. The FourPoints is also a converted communist-era high rise motel--never stayed there but it doesn't look to hot from the website. It really depends on what you want to see in Zagreb or what your purpose for visiting is. If it's tourism, the old town and zoo are closer to the Sheraton (although the Westin has good tram connections) and the Westin is closer to the main museums. The Four Points is a bit further out than the Westin.
By the way, Le Meridien is refurbishing a resort property on the Croatian coast a few miles south of Split (the old hotel Lav which was used to house refugees during the civil war). From the looks of things, it should be ready for next summer. That and the Hilton will be the only major chain properties in the Croatiain coast except for some Riu properties on the Island of Brac. But there are a number of smaller properties that are perfectly nice on most islands and in major tourist cities. Most tourists rent rooms from the local tourist bureau.