There are so many good suggestions for female supporting characters here...
Amanda Waller (in the long Suicide Squad run by John Ostrander) and Maggie Sawyer are the coolest ones to me

Lois Lane is would be my obvious choice.
But is she really a supporting character?
I would say so even if she had her own long running series in the Silver Age.

Peter Parker had such a large cast of potential girlfriend cast of supporting characters for a nerd!
Gwen, Mary-Jane, Liz, Betty...

And what about Aunt May! (not in the potential girlfriend category though... brrr.. that's why I listed separately from the other ladies).

Another cool idea for a weekly theme would be heroes' girlfriends. Gwen Stacy would be by number one choice!

Dan

Otherwise, Efrem Zimbalist jr's Alfred is a favorite, although really I haven't seen a bad Alfred. Michael Gough was the only constant in the Burton/Schumacher movies and he even did OnStar commercials for a while. And I really like how Michael Caine helped Christian Bale's Bruce Wayne retain his humanity while becoming the Batman.

Another good one was Ian Abercrombie from the short lived Birds of Prey TV series, as mixed as it was I think he made an excellent Alfred too.

For female supporting cast, my favorite wasn't in the comics but was in the comic related Batman Beyond- Max the friend of Terry's at school and in my opinion a much better match for him then Dana. Sort of how Chloe is better for Clark than Lana was in Smallville.

Meanwhile ... I wholeheartedly third the Etta Candy nom! She's the best. Always stuffing her face with candy but also always ready to mobilize and kiss some ass. On the machiavellian side, Amanda Waller comes a close second. Ma Hunkel has had some fun moments, both way back then (from reprints I've read) and again in Johns' JSA run.

Matt B. listed some great names but his best are all, as he suggests, arguably leads. Hey Bill, sometime after your supporting characters run is done, how about a couple weeks devoted to the "second banana" or "best B-listers"? That'd make room for characters who are too big to really be supporting. They stand out as much or nearly as much as the lead, yet they never quite managed to maintain their own title (though they might've had their own mini or failed ongoing).

How bout Night Nurse, most prominantly featured in Brian K Vaughan's Dr Strange mini.

In a perfect world someday I'd get to do a Day In The Life Of Alfred story. I don't generally aspire to write comics, at least not mainstream DC ones, but that particular idea nags at me. I keep thinking that's such an obvious hook to hang a nice little done-in-one Bat story on, and I don't think anyone's ever done it. Devin Grayson sort of almost did one in Gotham Nights implying a romance between Alfred and Leslie Thompkins but it came off kind of skeevy, and anyway I think DC's decided it got retcon-punched away or something.

Gough was certainly good. And though Caine did him proud in the movie, it was still Michael Caine to me. My ideal Alfred would be Alan Napier, but getting to play the part as written in the Justice League episode "Starcrossed." That's how I see him. Nobody could do the Alfred slightly-skeptical-cock-of-the-head and say "Very well, sir" like Napier.

As for female supporting characters goes:
Maggie Sawyer- Probably one of the best female supporting characters introduced in ages. She's strong minded, smart and just a good role model. I'd rather have her back in Metropolis running the SCU, but hey...she was great in Rucka's Detective run as well as Gotham Central.

Or how about Blind Alfred from Joe Kelly's Deadpool? Deep down, some of us want our Aunt Mays to be downright nasty.

I stand corrected, and hang my head in shame. :- (

How many times has Wayne Manor been blown up by the Kree? Or attacked by the Masters of Evil, only to have Alfred saving the expensive china?