I dont really have a favourite thing to possess, so Ill just list them as they come and give reasons.
I like possessing Sligs coz its fun to shoot things (

) and beat the crap out of Muds till the moment before they die.
I like possessing Scrabs coz its just fun. Although they cant do much, its still fun, mainly coz of the territorial thing between Scrabs; I like to instigate fights with other Scrabs

.
I like possessing Paramites coz of the communication thing. Commanding some other 'Mites to attack some Fleeches
really takes the pressure off me

.
I like possessing Gluks coz its fun to command a bunch of Sligs, especially when Im trying to get rid of a whole bunch of them, so I do this neat little trick where I make each Slig face another Slig, then I get all of their attention and get them to kill each other

.
Ive only done it once on a demo at the super market, but I possessed a BigBro with a Blitzpacker and mowed down a bunch of Sligs...which was fun

.