It is not possible to have 100% uptime, especially for a long time period. You can see however a lot of hosting companies that produce 99.9% or for a month or two. One of the best uptime ISP and hosts according Netcraft stats is New york Inernet. NYI.net is a FreeBSD and Windows service provider but when it comes to colocation or datacenter facilities they offer wide range of services. Their pricing is high even by NY standarts but I think that companies with large budgets that are looking for very reliable datacenter or colocation provider may consider NYI.net.
Netcraft say that the UK’s 1and1.com is one of the top performance web hosting companies. Other popular shared hosting providers that show good uptime are Godaddy, that has recently went for Windows OS, Powweb, Hostingzoom, Hostway and etc.
More information on Netcraft’s website.