The best place to train combat, whether it be strength, attack, defence, magic, or range is:
(Drum Roll)
The palace in Al Kharid. I know it may not seem much, and even I did not believe my eyes when I saw my attack and defence xp rising extremely fast, but this place is one of the best for fast experience. The drops may not be great, but there is a bank near by, and food can be obtained from the fishing spot near the bank.
Here is what the entrance to the palace looks like:

What You Will Be Killing
Al-Kharid palace guards
These guards are level - 9 in combat, and have 19 hit points, which means they give 76 xp per kill.
Here is me killing a guard:

I suggest that lower levels and pure rangers and mages bring along some food as these guards hit 1 and 2 quite frequently.
The only safe spots are behind beds and drawers and other stuff. This area is multi-kill, and other guards will attack you unless you block their path in some way.
Here is what the place looks like on the mini map:

Overall, this is a great place to train for players of all levels to train, and is the best place in f2p for ultra fast xp.