If you're looking for super aggressive hunter, many of the Monotrete puffers are a good bet and are generally between 4" and 8", especially the M. Palembangensis. T. Miurus (the congo puffer) is extremely aggressive as well. Many aggressive puffer species spend a lot of time buried in the sand waiting to ambush prey-- the Monotrete puffers in partufular. If you want a puffer you'll actually see once in a while, trade up to a 90g tank (or 4' 100g+) on Craigslist and get a Fahaka (T. Lineatus)-- they're $30 or so this time of year, grow pretty quickly to over a foot (max out at 18" or so) and kill EVERYTHING! A dot-line puffer is an even more active swimmer and is very aggressive as well, but expect to spend between $100 and $200, depending on your hookups. It's a cross between the lineatus and a pustulatus. ThatFishPlace.com had some "dot-line" puffers recently, and I think some MFK vendors have as well (I forget which ones).
IF you don't mind the lurking / ambush Puffers, then the Congo is probably the most aggressive out of that lot , but if your looking for a large active puffer thats aggressive, most would agree that you aren't going to find a more aggressive F/W puffer than the Fahaka, which is known most of the time to destroy anything that goes in its tank... although with that being said, you will need a larger tank, a 4X2X2 is normally the min. recommended tank size... around 125G?