yes, it is a solid defensive pivot, and it only gets 3hko'd by megadactyl. but why, in gods name, would you designate a team slot to avalugg, when there are a handful of mons that do the job much better, and you know, aren't shit in nearly every other aspect? it pales in comparison to other stuff that can do this, such as swampert, mega aggron, forretress, alomomola (who are all a huge momentum suck to megadactyl in comparison to avalugg), and probably some other mons that i'm forgetting that can do the job better. sure, it's a spinner with godtier physical defense, but it has shit spdef, a weakness to sr, and the physical bulk, in some cases, doesnt even matter when you get destroyed by the two most common physical attackers (as well as attacking types i guess) in the tier, and lastly, it's pretty much entirely outclassed by forry, who is pretty much one of /the/ best utility mons in the tier. i literally cannot think as to why you would use avalugg over any of the mons i mentioned. there's also a shitload of stuff in the lower ranks that doesn't deserve to be where they are, but i'd probably just waste my time in doing so.
one last point; while most of llamas points are right, a mon can be more viable in one meta than it is in another; just look at granbull, who risedose in popularity due to the immense amount of mons it can counter in the tier, i.e. hera, mien, hydrei, mega absol, etc., while there are not as many mons that justify the need of granbull in lower tiers. (this obviously doesn't justify the placings of claydol, lilligant, and friends but you know what i mean)
hopefully we can stop there.