I think this largely depends on which carrier provides the best coverage in your area, but say all three had equal coverage - which is the best?
According to a review "The Fastest Mobile Networks 2011" by PC Magazine published June 27, 2011...Verizon has the fastest network...
http://www.pcmag.com/Fastest-Mobile-Networks-2011#fbid=8uQ8gKWc3BV
But the iPhone 4s is using HSPA which is faster than Verizon's 3G. HSPA is like 3.5G