Pearl
Romance は風のまま
I have no clue how common Stall is on Showdown since I only play on cart, but a lot of Nippon dudes use it to grind the ladder and (regular) Tyranitar is one of the most common Pokemon on that team archetype for a lot of reasons, but mainly because of its ability to soft check threats such as Mega Gengar, Thundurus and Talonflame, all of which are among the most common Pokemon on offense and can be a little tricky to handle for stall (it's true that Chansey is a nearly perfect counter to Thundurus and that both mega and regular Slowbro destroy Talonflame pretty hard, but Mega Gengar poses a huge threat to both, which is where Tyranitar comes in)
Mega Tyranitar, on the other hand, is usually paired up with Excadrill on offense and its main appeal over other possible mega slots is being able to 1v1 both Mega Kangaskhan (most of the times) and Salamence, as well as working as a neat soft check to Talonflame, Gengar and most Thundurus-I variants. Personally, I'm not a fan of it, but I can see why people run it. It rarely pulls off a sweep by itself (if you want a mega Pokemon that can straight up 3-0 teams then give Zard X or Gyarados a shot), since it's kind of easy to revenge kill and has some pretty bleak match ups, but it can still pave the way for the aforementioned Excadrill and other dangerous threats such as regular Blaziken.
Too lazy to provide calcs right now, but you can trust me on this one (for the record, most Japanese teams I've seen with regular Assault Vest-less Tyranitar run 252 HP / 252 SpD Careful, so it definitely tanks at least one Focus Blast from most Pokemon)
Mega Tyranitar, on the other hand, is usually paired up with Excadrill on offense and its main appeal over other possible mega slots is being able to 1v1 both Mega Kangaskhan (most of the times) and Salamence, as well as working as a neat soft check to Talonflame, Gengar and most Thundurus-I variants. Personally, I'm not a fan of it, but I can see why people run it. It rarely pulls off a sweep by itself (if you want a mega Pokemon that can straight up 3-0 teams then give Zard X or Gyarados a shot), since it's kind of easy to revenge kill and has some pretty bleak match ups, but it can still pave the way for the aforementioned Excadrill and other dangerous threats such as regular Blaziken.
Too lazy to provide calcs right now, but you can trust me on this one (for the record, most Japanese teams I've seen with regular Assault Vest-less Tyranitar run 252 HP / 252 SpD Careful, so it definitely tanks at least one Focus Blast from most Pokemon)