yes you can decap a fp in melee range with any weapon, including - but limited to - ak47, katana, 9mm and knife (i also seen fps being decapitated at range quite a few times now so that too is possible, not sure what trigger it though).
berserker with chainsaw have the highest attack rate of all weapons in the game, no need to "reload" and a high damage per second - so it is easier for them as they just need to press primary attack until the head pop, the fp dies or the berserker dies
if not playing berserker or not having a chainsaw i think bullpup is the best weapon as it have a high rate of fire and lower damage per second that often let you decapitate the fleshpound without first enrage it.
my third weapon of choice to decap a fp in melee is the ak47.
after that it doesnt really matter as you will have to do the "one-attack-timing"-trick to decap it. probably 9mm or knife as they are fast low damage attacks and you always have them with you, but as long as you learn to time it right - any weapon will do, including katana.
To straight out kill it (if it for example hunt your partner) best weapons are probably xbow, nades (does "x4 damage" on fleshpounds), hunter shotgun, katana (preferable alt-fire), AK47, dual handcannons, chainsaw (preferable alt-fire) or L.A.W. (does "double" damage on fleshpounds, so it is actually half decent) - in that order.