The best experience I have had w an aftermarket battery has been a tie between Odyssey and Braille. Ruling out Braille due to high cost, Odyssey take the cake.
I have one in my 328, have had it since '08 and it is still at 98% efficiency as new. Literally less than half the dimensions of my OEM battery, a total weight of 21lbs, and 775CCA... My car has never died from lack of battery power despite a 33% underdrive pulley on the alternator (although it is an upgraded unit to compensate) and a not insignificant power increase to the sound system (Pioneer AVIC 7" double din headunit w 2x wired aux inputs, a combination of BSW Carbon Kevlar speakers, Focal speakers, and a JL Super Slim 10" Sub in custom 6"x16"x13" CF enclosure powered by a pair of small, light, but powerful 1000W amps).
The battery cost me about $400 four years ago, but it was worth every penny. I love dry cell units, and the combo of high power and low weight had me sold. It should easily last another decade.