F2P - 1) Spiders [Sec. Stronghold] 2) Flesh Crawlers [Sec. Stronghold 3) Moss Giants 4) Lesser Demons
#1 and #2 are your best bets due to the number of spawns
P2P - This largely depends on your gear you are looking to use, so not as sure. But probably safe with spiders. If you want money but a lil higher ban rate then go for flesh crawlers.
if you need gold charms i would do giant bat's when you're about lvl 70+, you don't have to eat a lot of food so you can bot a long long time. i got 85 str, 80 atck, 80 def with it in a short time.