It depends on what you are wanting to build a list for.
I use aWeber for my Free Opt-In Lists.
I use 1ShoppingCart for my Opt-In Lists for product and service customers.
The reason I use 1ShoppingCart for my product and service customers is because I have my gateway account set up with 1ShoppingCart as well, so I take all my orders through their service.
Once I have a customer in any of my 1ShoppingCart accounts, I can cross sell them with other products, etc. It's quite a seamless operation.
It's all personal preference though.
There are a ton of autoresponders on the market to choose from that best fits what you are looking to do.
Here is a helpful comparison list for you:
http://www.topemailmarketingsoftware.com/software/autoresponders/comparison
Enjoy!