SOFT bucking ALWAYS if you want better accuracy and consistency, only downside is that they wear quicker.
For a G36 hop up I think the best rubber/bucking is the madbull blue with a sanded off nub. Amazing results. literally put another 30ft straight trajectory over the stok JG one, with the mod of course.
For an M4 - everyone uses a normal cheap guarder one cause they are just that - cheap and they actually would be close enough to a stock TM rubber which is considered one of the best out there.
For my L96 - DEFINITELY brilliant with any type of gun though - W-hold bucking - I just get great range and accuracy with this. Best bucking ever IMO - I can't wait to get one for my m4.
Then for pistols or VSR's get the nineball or a stock tm is as good