Tarrasque or Tiamat

For my upcoming campaign, I am deciding which monster will attack the town my players are in, the Tarrasque, a 50ft tall monstrosity that destroys everything in its path, or Tiamat, queen of dragons, which the DMG says to use as a last resort.

Tarrasque is best for most campaigns. It’s canonically a wild single minded beast, it can slumber for literal centuries at a time and would attack anything in its sight.

Tiamat's a literal goddess and should be much more hyped up before being even seen. She wouldn’t attack any random town, considering how intelligent she is, she would probably strike at something much bigger like a vital city with a flock of adult dragons and her cults.

Tarrasque for sure, what level are the characters? Also, I agree with CCC, you definitely need foreshadowing before even seeing Tiamat, let alone fighting her.

Personally, I doubt "Tiamat, Queen of Dragons, swoops down and attacks!" will go over well as a random event. The Tarrasque also needs to be hyped up. Basically, don't just throw world-ending monsters at the characters for no reason, and with no warning.

I mostly agree with the above, but can the OP set the scene a bit for us?

At what point in the campaign is this happening? What level are the PCs? Do you mean for the characters to fight, or is this designed to force them to flee?

And what’s up with the town? Is it a little town in the middle of nowhere with nothing going on? Is it a major city? Does it sit atop some mystical site? Maybe without the knowledge of the inhabitants? I’m trying to understand why something might be attacking it in the first place, as the tarrasque doesn’t really need a reason, but Tiamat is only going to show up for something major.

Where does this happen in the campaign?

If this is the inciting incident, the thing that happens at the beginning to set off the adventure that's awesome and either would work well. The terrasque is more mindless and bestial however. It doesn't have intentions in the same way Tiamat does. So if you're building an adventure around the hunt for the creature that destroyed their village, then Tiamat would be the better way to go. 

If this is midway through the campaign the question you want to answer for yourself is why do I want the creature to attack the settlement?

Maybe you just want the cool image in your players' minds of the destruction. If they're not level 15-20 though they aren't going to be fighting either of the two monsters you propose. So what's the purpose of the attack? Figure out the purpose and you'll figure out the best creature to use.

Ask yourself when and why you want the attack on a settlement and it should help answer the question better.

Quote from Maruntoryx >>

Personally, I doubt "Tiamat, Queen of Dragons, swoops down and attacks!" will go over well as a random event. The Tarrasque also needs to be hyped up. Basically, don't just throw world-ending monsters at the characters for no reason, and with no warning.

 I agree, except maybe for blobs of annihilation. No lead-up really needed for those.

It is much more likely for the Tarrasque to pop up after some funky looking asteroid or omen in the stars or some other minor foreshadowing, than for a literal god trapped within the Nine Hells to swoop in and burn a town to the ground. 

Take Tyranny of Dragons. The entire campaign is a foreshadowing to the summoning of Tiamat, and the adventurers attempting to stop them. She isn't the kind of villain that just swoops in.

An ancient dragon swooping in is still plausible, though, and a dangerous threat. If you have access to Fizban's, you could use a chromatic greatwyrm for a unique and dangerous fight.

Quote from WolfeCriedPup >>

It is much more likely for the Tarrasque to pop up after some funky looking asteroid or omen in the stars or some other minor foreshadowing, than for a literal god trapped within the Nine Hells to swoop in and burn a town to the ground. 

Take Tyranny of Dragons. The entire campaign is a foreshadowing to the summoning of Tiamat, and the adventurers attempting to stop them. She isn't the kind of villain that just swoops in.

An ancient dragon swooping in is still plausible, though, and a dangerous threat. If you have access to Fizban's, you could use a chromatic greatwyrm for a unique and dangerous fight.

 That could work too…

Another thing you could use is a Colossus. From a dig site outside of town or earthquake 

Quote from naylortristan79 >>

Another thing you could use is a Colossus. From a dig site outside of town or earthquake 

 That or a blob of annihilation. They don’t need any reasoning!

What’s the context for this campaign? I don’t think we got any details like what level it’s starting and ending at, the setting or anything.