Lets get some numbers to see where we are on the realistic vs challenge opposition.

I submit that the first line should read "...no matter how easy, or hard..."

I typically go with realistic. However, the level of combat challenge often isn't an issue with my group, as they abide by the philosophy of "if a shot isn't fired during a run, then you did it right". This may change as they have recently acquired a mortar. But there you have it.

It also depends on the scope of the game. If a particular shadowrun group is regularly mowing down security guards and the security company is not meeting its contracts, that security company is quickly going to get canned and replaced by a stronger, more aggressive security company (i.e. Knight Errant), making the overall difficulty of future runs significantly harder. If there is a rash of Shadowruns against a particular company, that company might start to reallocate its "security assets" against physical incursions. Likewise, if there have been a lot of infiltrations recently, the level of corporate paranoia might increase (now using retinal scans instead of name badges).

Note that any opposition can be challenging, given the right circumstances. If you are having trouble with runners greasing your security guards, use ambushes, flanking, and other tactics (a well-placed grenade can do wonders). If the opposition is too difficult, you can have the runners captured instead of killed (hey, the runners should know to run if they in over their heads... cutting and running is a time-honored shadowrun device).

The runners also have to be sophisticated and professional enough to learn when to turn down a job (and when I plan my runs, I try to give them that option) if the heat is too much, or at least ask for more pay. Likewise, a Johnson/fixer isn't going to hand over a cakewalk run to a group of experienced shadowrunners... why pay more, when paying less to a newbie group would achieve the same results?

Of course, if the opposition is hopelessly overwhelming, the fixer should've found a better team, and the PCs probably wouldn't get offered the job in the first place.
And if the team is too good for the job, then they probably expect too much money, and the fixer should've found a cheaper team, and the PCs probably wouldn't get offered the job in the first place.
So my runner teams do usually end up meeting resistance that is approximately appropriate to their ability, but that's because as they get better they build a reputation and start being offered more difficult and higher paying jobs.

But as soon as they go outside a fixer or someone who knows their level of ability, or do something more independently character motivated, they're getting a response appropriate to the situation, character-abilities-be-damned.

Always realistic. Setting them to scale with my group just makes the run seem more DnD-ish.

Realistic as in "Realistically, no corp would spend half a million nuyen on implants for each security goon" or as in "Realistically, no corp would be stupid enough to rely on security goons who can't stop any newborn runner ?"

I have to go with Moon-Hawk on this one. Generally speaking I like to have the opposition be a challenge for the team, at least on runs that they themselves didn't think up. This means that the enemies should be of a calibre to hurt the party. and that means sending the party somewhere that the defenses will be a challenge.

So I guess I would have voted "realistic but still a challenge."

my gm would find a way to balance any run-- if the security amounted to a few boyscouts with pocket knives there would always be an extremely challenging puzzle or maze we had to figure out... it also cut down on min/maxed characters-- being seperated from your crew and trying to figure out how to shut off some deranged mages's trap with a 1 int was not going to go well for your character.

In the games i've played we usually found reasonable opportunity. Sometimes that had been hard to overcome, sometimes easy. But most of the time it had been fair fights. If both sides (GM as well as players) use cover etc. a fight can be very challenging, even if one side is "weaker".

Gangers should be pitifully easy, but have quite a few tricks up thier sleeves when on own turf.

Depends on the gang. A fight against the Ancients or the Spikes would be a lot different to one against the local bully-boys.

If the runds calls for low end security gruads with a taser then that is what it gets. On the same hand if you are trying to break into Fujicorp, be prepaired for heavy firepower, cyberd patrols and the such.

It depends on the individual fixer or runner. Some fixers will try to get as much of a cut for themselves as possible, some runners only take runs that pay well. Some fixers go for overkill, more concerned with his stable coming home in one piece with the run completed than getting as much a cut as possible. Some very experienced runners will go for a 500 or do it pro bono.

I keep the opposition at the site fixed no matter how experienced the runners are. The only time I add to the opposition is when the run is somehow compromised.

I`m in the realistic regardless boat with so many others.

That Shiawase compond that looks like nothing special and is located outta the way only has a small sec team. But they are enhanced and well trained because there is a reason Shiawase was keeping the facility looking low key. The big flashy building in the middle of down town..sure they have a HTR team and a steady Astral and Magic sec presence but most of the main sec team is Joe Av sec - guard, trained to be good in PR, loyal to the Corp and how to shoot a random intruder if they are trying to nick stuff....and thats about it.