Lvl 28. Delrith.
At your level, I would say killing Experiments ( Level 25 - 100Hp = 400exp per kill ) near Canfis.
The best monster to kill in Runescape would be the King Black Dragon.
At that level, you might kill cows and chickens.
in the deep wilderness against greater demon level 92
Waterfiends
That would be a goblin.
level 25 experiments are weak but have 100hp giving you 400 exp per kill - located near canfis, morytania.
Easiest monster in Runescape would have to be a rat, man, or goblin. If you want something that is easy to kill that gives good experience that would be rock crabs which are member only. They are located up in Rellekka.
train kill stuff
Gargoyles, but you need a quite decent slayer level to kill them, people say they drop more often than gold there.
Corporeal Beast, Mithril Dragon, Kalphite Queen, King Black Dragon, Tormented Demons, Chaos Elemental.
Dragons. Their hides go for loads of coins, expecially the Black Dragon Hides.