Well, Rapid Spin and Defog are both status moves, so you'd want a Pokemon who's either fast, or has great Def and Sp. Def. But to me in particular, I'd choose a Pokemon with great defenses, with satisfactory speed. In that way, you can withstand enough attacks to use it, but at the same time, not be the slowest on the field. The problem I think with the fast tactic, some priority moves can play against them.
I'll start off with your 3 suggestions.
Forretress: good Atk + Def | bad Sp Def and Spd (1 weakness)
Donphan: great Atk + Def | bad Sp Def and Spd (3 weaknesses)
Avalugg: better Atk + Def | bad Sp Def and Spd (4 weaknesses)
So overall, each of these Pokemon are similar, and yet each of them can learn Sturdy. But I'd particularly chose Donphan since it has a good move-pool, as well as having slightly higher Sp Def and Spd than the others. But, any of these 3 can work.
If I were to make a Suggestion, I think the best choice is Blastoise as it provides everything your looking for and more. But, If you aren't able to obtain this starter, I'll suggest Claydol. Both of these Pokemon are for your physical + Rapid Spin Pokemon. Claydol can give you great STAB moves, and it can also learn Power Trick to switch it's defense and attack.
If your looking for a Pokemon to learn Defog, you can try Braviary. Most Pokemon who learn Defog don't have a good overall balance, but I think Braviary will be a great choice.
Hope I helped! :)
