Traditional active freestanding subwoofers are the most popular choice for home cinema systems. They provide good performance, flexible positioning and value for money. The size of the subwoofer should generally match the size of the space it needs to fill, but bigger subs with larger drivers and greater cabinet volume do produce lower frequencies, so typically bigger is better. For ultimate bass performance multiple subwoofers are often used.
If it's undesirable or not possible to position a freestanding subwoofer in the room, then why not consider a discreet in-wall or in-ceiling subwoofer.