Like modchips, this is individual to the user and what he wants to do. The big three dashboards (in my opinion) are Evolution-X, Avalaunch, Media-X-Menu
Evolution-X - A very simple and easy to set up dashboard, it has many hard ware features, such as flashing the bios, formating the hdd, ect. Its an excellent beginners dashboard
Avalaunch - A really cool dashboard, although it doesnt have as many features as Evox (Evolution-X), it does do some neat stuff. It will play background music, has a built in file manager, gives previews of games, lets you download gamesaves directly to your xbox, no ftp needed. Ect.
Media-X-Menu - Along the same line as Avalaunch, its visualy appealing, but I dont know much about it.
You can also use UnleashX, XBMP, XBMC, and lots others can all be used as dashboards.