The best storage for data is a database. It can get as big as it needs to and is easy to query.
If that isn't an options then the other defualt report formats (ATML, XML, HTML, TXT) all display the same information. Text will be the least memory intensive because it doesn't have any markup language. However, it is the worst to read. XML and ATML require style sheets to format the data. So when you send the reports to someone you have to include the style sheet for them. HTML uses more memory because it needs more tags but is easy to move to other people and looks somewhat decent.
Hope this helps,