I still believe in this game that the UEF can turtle best, because their short range artillery, it fires quick and it can't be ignored.
An extreme version of turtling is to forgo building mobile military units entirely and instead use all one's resources on an impenetrable defense line and large quantities of artillery and other indirect fire structures. One advantage of this strategy is that artillery does not require resources to fire. This means that even if the player's resource generation and unit production are disabled, they could still fight and possibility win.
The choice of faction has a large impact on the feasibility of this strategy. The UEF has the fortified artillery station which, when used in large numbers, can be a very effective defense on it's own and can also help soften incoming armies for point defenses. However, if playing as UEF one will need to send engineers out to build sacrificial radar stations in order to target far away bases, this can obviously be challenging with no military units to guard the engineer.
Sacrificial radar stations are not needed when playing as the Cybrans if the player researches shell camera. However, the Cybrans do not have a medium range artillery to soften incoming armies, meaning their point defenses must be able to take waves of experimentals without breaking.
This strategy does not work nearly as well, if at all, for Illuminate players as that faction does not have long range artillery. An illuminate player would have to rely on nukes and building tactical missile launchers close to the enemy base.