I don’t know if it’s the most cost effective but http://bluehost.com has worked well for me (also owns HostMonster.com). Good Linux based hosting, good price & service etc.

If you are going to use different domains and want them to not re-direct to a shared SSL hostname, for the ecommerce sites you will need seperate static IP’s & SSL certs for each site. So that may bump up the level of account you need.

The SSL Cert is tied to an IP address & hostname if you are using just 1 domain and have some sub-sites you can get away with 1 static IP and SSL cert. (Liquidweb’s pro plan will work for this)

For somthing like this I would recommend either:

Media Temple (http://mediatemple.net/webhosting/gs/)
or
LiquidWeb (http://www.liquidweb.com)

Yes, they are more expensive than some of the dirt cheap hosts (Bluehost, Dreamhost) they offer far better support (you can call someone 24hours a day and they speak english) and better uptime.

I have/had sites with all 4 of the providers I have listed, I still use Dreamhost for my hobby non critical sites but I have moved all of my client sites to Liquidweb.