The fan favourite is always soft drink, but I think there's some important caveats to remember:

If you haven't got any food in your stomach, it will empty much more quickly.

Cold stuff is good as it inhibits digestion by reducing the effectiveness of various enzymes, leading to a lasting bloat, but it is also difficult to consume large amounts of anything cold.

Raising pH also inhibits digestion, again resulting in a much longer bloat, as long as you've got a decent amount of food in there. Soft drinks are always acidic (dissolved carbon dioxide) and will quickly undo any attempt made to neutralize your stomach acid.

If you want to feel as full as possible, you must avoid burping which can be easier said than done. Certain foods will foam up and capture gas, preventing it from being burped up as easily. The crowd favourites among these are bananas and dairy.

Source: my tortured guts