Whenever you see a commercial for an ISP who make claims on how fast their Internet service is, what they are telling you are the maximum theoretical speeds which you will virtually never reach (if you notice in the small print on commercial says your actual speed will vary). But if you want to see h0w the ISP’s stack up overall then check out this article on PC Magazine which looks into this topic.
Their conclusion finds that Verizon FIOS is the fastest for casual browsing. What’s interesting is their seems to be a clear separation of cable and DSL with almost every cable provider faster than every DSL provider (save one exception). Of course actual speeds can be determined by a variety of predictable (location relative to a hub) and unpredictable (lines down, overall network usage) so there cannot really be a complete definitive winner.
The results of this article have been questioned, but overall it seems to be pretty fair.