What free to play creature drops nature runes often?

There are several monsters that drop Nature Runes in the Free to Play area, ranging from different combat levels. I'll list them below and include the drop rates of the Nature Runes, ultimately what you end up choosing to collect Nature Runes from is up to you.

Hill Giants (Level 13-28), drop Nature Runes at a common rate and they are dropped in amounts of two-six. These guys can be found in the dungeon below the small house south west of the Grand Exchange. Take a bronze key for easy access. Probably the easiest to kill of all the listed monsters.

Flesh Crawlers (Level 28), found on the second floor of the Stronghold of Security have a rare drop rate, however they drop them in amounts of five-ten. There are a few rooms that have only the flesh crawlers, found in the far south-eastern corner of the level.

Hobgoblins (Level 28-47), drop nature runes at a common rate in amounts of four per drop. These guys are pretty easy to defeat and also drop Limpwurt roots, which may be helpful.

Moss Giants (Level 42-44), drop Nature Runes at a decent rate in amounts of six and aren't that difficult to defeat. These can be found on Crandor and in the Varrock Sewers. Though the Crandor ones are probably the least likely to have other players there.

Cockroach Workers (level 56), drop Nature Runes at a common rate. However the amount dropped varies between one-twenty, most likely being on the lower scale of that spectrum. They can be found in the Stronghold of Player Safety, best accessed via the rift near Edgeville bank. They are found on the middle level, surrounded by Cockroach Soldiers. They are not aggressive, which is a bonus.

Ice warriors (Level 57), drop Nature Runes in lots of four at a common rate. They can be found in the Asgarnian Ice Dungeon and are near some Ice Giants. They are aggressive, so they may become annoying after a while.

Ice Giants (Level 58), also found in the Asgarnian Ice Dungeon drop Nature Runes at an uncommon rate in amounts of six. Being a higher combat level than all the other options these would only be efficient if you are a higher level.