What maintenance-free battery model is the longest lasting on our B5 A4s??

I've had my DieHard Intl for 4+ years and still going strong

Same here, in both A4's and my old Saab.

2nd Die Hard. I had a 3 year old battery get drained while my car was sitting in the cold...
 

when I had it apart for maintenance. We attached a battery charger to it to trickle charge overnight. A friend was adamant that the battery was done for. I've been using it since (last 9 months) with periods of 10 days sitting. The car still cranks strongly. I give Sears batteries a huge thumbs up.

just pick whatever is cheapest with the CCA needed that has the longest warranty, and fits...because
 

most auto batteries are made by just three manufacturers: Delphi, Exide, and Johnson Controls Industries. Each makes batteries sold under several different brand names, which Delphi makes ACDelco and some EverStart (Wal-Mart) models; Exide makes Champion, Exide, Napa, and some EverStart batteries; and Johnson Controls makes Diehard (Sears), Duralast (AutoZone), Interstate, Kirkland (Costco), Motorcraft (Ford), and some EverStarts.

So, bascially, run down to your nearest Sears, Autozone or Costco to price compare the "same" battery.

DieHard. The one that came with the car lasted six years. The new one is going strong.

a comment....
 

Here are the ones I prefer:
1)Diehard Gold
2)OEM Factory replacement(Moll or Bosch)
3)Optima
The above mentioned items are not in any order of preference.
Comment: I've found the higher quality replacement batteries have seemingly better breadth and depth of charge over the standard discount units(ie Costco, Pep Boys, etc.). We've got a Costco unit in one car and its been replaced under warranty once already. I've had similar experience with the lower tier batteries from Sears and Interstate. I should note that when the families cars are not in use they are hooked up to a Battery Tender Jr. So, its is not as if they are abused or neglected.
Regards

The Group 48 Die Hard Int'l is the same battery (actually fewer CCA) than the Interstate MTP-93 and
 

the Costco Group 48. I mean same. Same look, same drain tube with the same packaging.

Except Interstate charges extra for the drain tube while Sears and Costco provide it for free.

100+ for the Sears and Interstate. $60 for the Costco.

i have a Bosch one from Pep Boys. No probs. Been in there 3yrs, fits like a glove.
 

just had to buy a universal mount to hold it down.

Lynx brand kit from AutoZone, part #00203

you're just buying the shell and its name, imo. Johnson Controls makes the core of most of those.