I used to use a lot more icons. Was on GJ for a while, when it had a 2000-icon limit, and I got up to about 200 icons, all of which were used at least semiregularly. So the question of which one was default didn't matter too much.

For a while on LJ I had a Plus account, so 15 icons. I tried to have at least one for each fandom I commented in regularly, and every so often I would delete them all and put a new batch in rotation.

I also used to have a thing for putting my name on my default icon. Have backed away from that since, although I still use an icon with my name and a picture on other sites (DailyMotion, DeviantArt, various forums) where you only get one, so it's more like an overall avatar than a reflection of your mood at the moment.

I downgraded to the 6-icon Basic LJ account a while ago, and ever since then I've actually been going the low-maintenance route, having a generic default icon and not bothering to upload others. To that end, I'm kind of addicted to jai's beautiful stock icons. On DW I cycle through a variety, but tend to use something classy and stock-y as my default.

And my default icon on every account right now is a festive one, because that's how I roll when December comes around :3

...well, that was rambly and tangential! Hope some of it was interesting ^_^;