This all depends on wether or not you are a member or non member, and your combat level.
Members: Rock crabs are probably the best place for anybody lvl 3-90 to train. because they have such a high hp level at a low combat level.
Non Members: I would recommend training on things that cant hit you very well when your a level 3-10, such as rats, men, goblins. 10-20, train on cows and pick up the hides for money. 20-30, al kharid guards are very good because they attack you if you attack one of their 'brothers'. 30-40, you can train on zombies and skeletons because they have a decent amount of hp. 40-70, train on either hill or moss giants, preferrably moss giants because they have more health but its multi combat in the sewer where they are. 40-70,Train in the security stronghold in barbarian village 80-126 either train on lesser demons or in the security stronghold