What battery is everyone using

Uptill now I've only bought about 5 used gel-batteries over time for various cars and they've all treated me good... except for one that is, where indeed the previous owner had let the battery go flat a couple of times (too many).
Turned out he used the battery for powering the led-lighting in his garden and one time he forgot to turn them off.
The battery shows full when checked, but it has no power when trying to crank an engine.
I'm currently trying to get it to hold a charge again by hooking it up on the slowcharger everytime I'm in the garage.