There was a very good article targeted at Doppelganger's as a player character race that might have some helpful information.
Before gaining Polymorph Control:
- Medium-sized humanoids can wear all armor and use weapons.
- Hill orcs are guaranteed at level 2, and are strong monsters
- Elf and dwarf mummies are reasonably fast, and do not have to eat, making them if you want to wait around and sacrifice at an altar
- Doppelgangers can start polymorphing into mind flayers with a reasonable chance of success at level 6 or 7; the brain-eating attack will make short work of most monsters, but is useless against those without heads or brains, and downright dangerous against monsters with passive attacks. Note that mind flayers will not be stoned by attacking cockatrices; the tentacle attack is instead turned off.
- Although not guaranteed until level 8, a level 5-7 character has a reasonable chance of polymorphing into a basilisk. They are essentially stronger cockatrices, with the same passive attacks. As such, they can be very handy for getting through the Gnomish mines. Note that nutrition can be a problem, since you can't eat statues. Also, the similar asphynx may look appealing, being faster, but it can carry very few items, and cannot pick any up at all.
- Blue jellies have a powerful passive attack and are a guaranteed polymorphable form at level 4. Unfortunately, they are also immobile and their lack of carrying capacity will result in nutriton burn if you do not first drop ALL of your equipment. If a non-cold resistant monster attacks you in melee they will take damage and heal you, possibly also causing you to split creating tame blue jellies. This can be useful for survival in the Gnomish mines, and the tame jellies can be abandoned and killed after becoming untame as a source of cold resistance.
- Gargoyles and Winged gargoyles can sit and lay eggs that will hatch has gargoyles, winged gargoyles cannot wear body armor but normal ones can. Both forms also have excellent natural AC.
- Baby dragons are guaranteed starting at level 4. They can lay eggs, which will then hatch tame. Unfortunately, the larger disparity between the base levels of baby and adult dragons in SLASH'EM makes getting pets dragons from baby dragons harder, although not impossible.
- Demons have low natural AC, good attacks, and are a source of powerful pets through demon gating. The weaker demons become guaranteed polymorphable forms before a doppelganger gains intrinsic polymorph control at level 9 as well.
- Chasmes can attack quickly and can repeatedly put foes to sleep. Great for killing shopkeepers (though this will come at the cost of -5 alignment if you kill the shopkeeper without having him/her become angry with you due to not waking up).
After gaining polymorph control
- Giant shoggoths are about the most powerful form, making short work of just about any monster, including One-Eyed Sam. They are stoning resistant, can eat metal, are quite fast and have teleport control. Unfortunately, they are also blind, and have no hands. Thus, they make poor traveling forms, but are excellent for problem-solving.
- Crystal golems are too large to wear body armor, but intrinsically possess magic resistance, reflection, fire, cold, sleep and shock resistance. They will always have 300 hp as well. While their melee attack only does the damage of your wielded weapon/bare-handed combat abilities, they also possess a random breath weapon, possibly including disintegration.
- Skeletons are somewhat slow, but are reasonably strong, stoning resistant, have a good natural AC and can wear all armor and use weapons. The slow speed can be offset by intrinsic speed or (better) speed boots.
- Master mind flayers are stronger than normal mind flayers. Note, however, that in SLASH'EM they oddly have fewer brain-eating attacks than normal mind flayers.
- Dragons of any color have excellent defensive and offensive capabilities and by sitting they can lay eggs, which can then grow into pets.
- Black dragons have disintegration breath, which is instadeath to most things.
- Xorns and earth elementals have the ability to travel through walls. Stealing from shops is easy and escaping into the middle of nowhere to heal up is a great bonus.