I think wadcutters would be the loudest "poppers".
H&N Crow Magnums! They're the loudest pellets I've ever heard. They completely wreck everything they hit. Problem for me is I have a hard time hitting anything with them. Terrible accuracy in my springers.I used to use wadcutters when space was tight. They were safer to use and made a nice pop! I've since moved to the country and I use domes because I get better accuracy and energy at longer ranges. I can still hear them hit but its more of a thud than pop.
You don't need that much gun or lead for HOSP's! Lol. If you want quiet for general use/pesting, .177 JSB 8.44gr or H&N FTT's at 12fpe to 15fpe will do the trick. Between my soft tuned Marauder, full power HW97 or HW95 w/Weirauch can, many critters have been taken without disturbing people or animals in the area. These pellets at said powers allow reaching out to 50+ yards accurately, but really any domed pellet will do at shorter ranges and/or lower powers given good placement. As for loud, once you get around 30fpe or better in .22 or .25 about all pellet impacts get noisy. Big hollow points will make a noticeable pop, even at lower powers. It's been a long time since I've used .25cal on pests or game, but remember impacts at 50 yards being louder than the muzzle report/action noise while using JSB 25.4's, being shot at a mild 708fps (+/-) from my Remington VTR 725 break barrel.