Well, Avalugg is super slow, which i don't recommend for playthrough.
Although it's Def is super, but it's Sp. Def is the contrary so it's too risky.
Sure both Cetitan's Def & Sp. Def are below average, but it's super HP is covering them.
Avalugg's movepool is great tho', many choices here as well.
You can take advantage of it's stat by teaching it Body Press and Avalanche.
In the end there's no single answer for this, it depends on your current team, their movesets, and what's your vision for the perfect team.