Welll, really, with Arcologies being fully-contained cities (Well, in theory), sticking them in nowhere'sville works out pretty good for 'em, a slong as there's a reason to be there. Omnitech-Debeers might have an arcology in a diamond-rich area, for instance. Help keep out warlords and SINless riffraff, keep an eye on your workers to make sure nothing goes missing, allows production on-site for maximum profitability, and so on.
Every decent sprawl should have a few... not Renraku Arcology big (That thing's a MONSTER!), but big enough. Tokyo has nearly a dozen, just in the bay, for instance.
Certain corporations, like Proteus, Renraku, and Shiwase, are more likely to have them than others, like Ares or Wuxing.
You can also give them some personality without too much work. In Philadelphia, for example, the sprawl I run a game in, there're three of note.
The first is "The Black Box", an arcology from MCT. The population is actually quite low, with the bottom floors all being construction areas for assorted industrial products, the medium levels having residential and entertainment/shopping areas, and the top, of course, being for execs. The Black Box isn't a typical pyramid, but rather a big ugly black box with a skyscraper stuck on top. Efficient, yes, but all kinds of ugly.
The second is the "Shia Pet", a SHiawase arcology done in a more traditional style (Of course!), but to highlight their growing enviro-friendly status, they capped it with an urban garden and lined the top of the place with moss and vines. Unfortunately, this wound up looking like a bad green wig, stuck on an aging actor, not unlike a Chia Pet. The name stuck, and Shiawase's reputation took a hit. Of course, there are those who say that Shiawase screwed up on purpose, in order to make themselves look foolishly incompetent, to draw attention away...
The last is Olympus, a Horizon arcology that's still under construction. Originally, it was a Renraku facility, to replace Renraku City, a campus-like Pseudo-Arcology and teh home of the "Tower of Babel", but in the wake of the Arcology Shutdown in Seattle, and the fallout from same, Philly and Renraku came to the conclusion that maybe they should quietly Not Do This Thing. It lingered in the wake of the Crash, but then Horizon stepped in during their expansion and snapped the place up for a fraction of the price. From there, they started expanding and re-configuring the place, making it the east-coast home of their entertainment empire and home of market research for the UCAS. Olympus is still in construction, and will be for probably a decade, but some parts are already up and running, giving ample targets for Shadowruns.
Taa-daa. Three Arcologies, similar yet different, reasons for existing, local nicknames, some plot hooks... they're darn handy, but can be overused and often don't make a lot of sense. Ares, for instance, barely has a presence in Philadelphia, mainly using the city as a place to launch runs against other companies, knowing that, locally, there's not much fear of reprisal. When there's only ten thousand Ares workers in the city, there's not much need for the huge cost of building an arcology. Aztechnology, meanwhile, has hundreds of thousands of workers, but for all kinds of subsidiaries. It doesn't make sense to put them under one roof, so while there are some Azzie facilities, they'll be under other names (StufferShack, NatVat, All-American Media, and so on) The only "Aztechnology" facilities, for example, are the Aztechnology Tower, a 'community outreach' Temple of the Sun, and a private Aztechnology 'campus' out in the suburbs with a second, smaller, pyramid.
