Considering the nature of Digivolution, 2-way as opposed to 1-way exhibited by Pokemon, I feel that it should start with Champion level and build up gradually, with Rookie available as a temporary thing as a result of a 'nerfing' combo. But there is the issue of digivolution trees to manage...
As a given, most of the main canon ones, particularly from the first 2 seasons, should be included. But I will still include some in my favorites...
As for preferences:
Agumon > Greymon > MetalGreymon > WarGreymon
Guilmon >Growlmon > WarGrowlmon > Gallantmon
Fanbeemon > Waspmon > Cannonbeemon > TigerVespamon
[To Be Edited]