"what is the most reliable and cost effective ISP for a web designer"
"who is the most reliable and cost effective ISP for a web designer"
This depends on your scope of work and requirements. All ISP's are good in their own perspective. If you are looking for a good world class standard then http://www.justhost.com/
is the place to look at 1st.
In Zimbabwe most ISP's do not give control to the developer & their systems are semi dynamic. I would encourage you to do your own research if you believe that local is Laker and you want to develop web apps that can inherit the ZINX speeds :-) . Start with Africa Online,Ecoweb,iwayafrica,Utande, YoAfrica and Zimbabwe Online.
YoAfrica is my choice. I have delt with them since they had CABS in their name. I respect their quality of service & should you have issues that need a hand they are related to WEBDEV which helps in web development & hosting etc. Another place to check is with Utande.These guys have been around for a while and have a good muscle in-terms of capacity & experience.