Thanks to Greymon X Anti-Body special digivolution on top of any Greymon for 0 cost, is very easy and cost efficient to make his inherited effect active for a Lv5 Digimon with Greymon in its name but that is only the baseline of how crazy this effect can become.
Still on the more traditional side, red has 2 very notable Digimon that can digivolve on top of others of the same level, them being [BT7-011] BurningGreymon and [BT5-015] MetalGreymon Alterous Mode.
Its worth mentioning the line that lets it digivolve from greymon is the same used for armor evolution meaning u can put this on the bt08 greymon (black) then digivolve to say bt1 metakgreymon now that its red giving you a 9k lvl 5 blocker with protection to make sure you can get to the next turn