I'd say rock golem and addy golem and slimes. or if not slimes,  then mith golem

Entirely depends on what you're after. Some are better money, some are better exp, others make for fast points.
It also depends on the master you're using and your rates. Wilderness tasks have different areas/npcs that can be hard to reach or otherwise less efficient. Lastly, it can depend on whether they have items you're after
Check out the Slayer page on our wiki. It has a lot of info on which tasks are given per master and what npcs drop. Also try browsing through the npc droplists in-game (under the stat tab - guides - Bestiary). If a slayer npc isn't in the bestiary, go near them in-game and the droplist will automatically get updated.

Best of luck!