It depends on what you're going for.
-Ferrothorn can set up hazards while eating moves for breakfast. It also does damage using barbed wires if an enemy makes contact.
-Roserade has high SpA and can be used offensively while also doing DoT with toxic. It has healing moves in Synthesis and can be used to stall out turns.
-Appletun has a sluggish Spe stat but makes up for it in bulk. In addition, it can g-max and gain it's exclusive move G-max Sweetness which has a secondary effect that heals your party's status conditions.
Ferrothorn has the best secondary typing due to having 2 weaknesses and 10 resistances. Appletun's grass/dragon typing has 5 weaknesses, a 4x weakness to ice and only 4 resistances compared to ferrothorn. Roserade meanwhile has 4 weaknesses and 5 resistances.
There's no real answer to this as it's preference based but if I were to choose, I'd pick roserade due to having a decent typing and having the most offensive capabilities. Ferrothorn is more suited for defensive play, combined with its ability to set up hazards. Appletun doesn't really have much going for it other than bulk, so it's probably the worst pick on the list. Do what you will with this information, good luck on your team