Competitive, Tyranitar. I mean, it's kind of obvious I'd say that, but there's a reason that this big beast is in OU, almost constantly since its release.
This is why:
- Amazing ability in Sand Stream that buffs its Special Defense to massive levels
- Access to several kinds of moves, leading to many functions. For example, it could be a Dragon Dance Sweeper or a tank, or a Mega, or a Lead + Pursuit Trapper, and so the list goes on.
- Fantastic weather lead, which can both support other Pokemon as well as sweep on its own, given the right conditions
Unfortunately, all that comes with a heavy price. Or many prices: Poor health, atrocious typing, given how common Fighting and Fairy and Steel and Ground and Water (see where I'm going with this?) moves. A strong STAB from these moves forces the slow monster to pull back, if not OHKOing non invested versions.
While for ingame too, Gigalith is kind of hard to obtain (not if you have someone to touch trade), so with that in mind, it's safer to say T-tar is better again. Not by too much, but still.
However, there is still hope for the pure Rock monster that is Gigalith.
Sturdy + Custap Berry let's you pull off at least one Stealth Rock and cause massive damage by Explosion (that's how I lost my Battle Mansion streak, so I'm sore about that: Custap Explosion is a merciless bane)
and it's still good with great attack and defense, EQ and Stone Edging a fair number of lower tier Pokemon.
It depends on how you choose to play, but it is obvious T-tar wins this one.