If there was such a time, it would be known as a simpler time when every default file had one name. Sure there would still be millions of extentions - .htm, .html, .php, .php3, .asp just to name a few. But simpler times none the less.

Those simpler times remain mainly fiction however as one secret war still wages across the web server industry - what to call the default document in each folder? Index.htm or default.htm (filenames excluded, they could be anything).

The classic analysis
The classic analysis would be to look at which is the rightful winner. Index.htm has been around for a long time - often it is the file presumed to exist on a server. It seems quite a good name for a default folder file as when it gets down to basics of it, the homepage, provides an index of the files and folders contained.

Default.htm is now quite widely accepted indeed there are many situations where Apache is recognising default.htm files and not index.htm. Perhaps there is no winner here one is popular, one is classic, just like .htm and .html.

The scientific analysis
Science is here to solve all our little problems. So why not look at it from a scientific point of view. Index.htm has some relevance to the name so people should understand it. More importantly, it is a shorter word than default taking up fractionally less space and perhaps even fractionally less processing time.

Default on the other hand is longer but comes first alphabetically so if you are scrolling through a list of files if you have many in one directory when you will be able to gain access to default first. It is also quite self explanatory so usability is also good here.

The real world analysis
When all else fails, head onto the internet and find out what other people use.

Sites that use index:
# Almost the entire Worfolk Online network
# eBay
# Disney
# BBC.co.il

Sites that use default
# A few parts of MSN
# Microsoft.com

Conclusion
There is no clear winner. Even with the real world analysis although the majority of the sites I managed to find a default filename were index, with heavyweights like Microsoft behind it the analysis can't say anything clearly.

It all comes down to personal taste - after all, it is server side so whatever you feel comfortable go with that. Personally I prefer index. It's short and sweet.