Pretty sure the best is whatever you can potentially 1 hit gives best per hour

Example (at 99 str but still pertains to lower levels), higher/same level type mobs are slower to kill then lower levels.
Black dragons gives what, 2000-3000xp? and takes say 30 seconds to 90 seconds to kill
Guards you can 1 hit and give 400-500xp.

so potentially you can kill 20-40 guards per black dragon.

Even with 10 kills per, its still almost double what a single black dragon gives.

Same example for ranged, killing a black dragon sometimes takes 50+ arrows (non rune) for the 2500ish xp, or killing rats at 1-3 arrows each for 300ish xp each...Much faster gain, just less loot, just pick what you find fun.

In the wild you have a number of spots to pick from: Thugs, druids, red spiders, darkwizards, skeletons, hobgoblins etc.  Each have their own risk, more pkers, further out and so on.

Outside of wild if you plan on afking, or semi afk / picking up bones/runes just remember most aggressive mobs stop attacking when you reach double their level +2 (i think) so giants are lvl 37, they stop attacking at lvl 76 if its +2.