Water shifters might be one of the best ranking things there is. (Though I might say a mariner wood elf would be better)
Especially if you go defense and get a regen form.
Water shifter forms were (not sure if still are) immune to drowning attacks. Khardraths has the elemental plane of water. Once you get one form, you can rank without getting hurt. (Plus you have full ABS in there)
I had hippo, great white, komodo.
Kill something as great white, shift komodo, heal fully in a few hours, kill another thing with great white. Get 1000 xp.
Repeat.
I was killing sahuagin scouts with the hippo at the low 30s with a group, getting something like 2-3k a pop. But without a group you just go to a non-hero area and rank, and you'll still do very well.
Plus water areas can rank you pretty much from 1-51, and allow you to avoid annoying thieves/assassins in the process.
Only real problem is water forms sort of suck as you are constrained where you can use them depending on the form.