Abyssal demons - these are what most people aim for, the chance of getting a abyssal whip for free! these tasks are great but dont expect you will get a whip drop every task it is over 1/500 in chance but still it could happen.
Dark beasts - drops the dark bow, used to be over 1m but now around 400k but with the ring of wealth update these do tend to have a large variety of drops. I have recieved many rare herbs such as torstol in decent amounts aswell, around 30 in each drop, and the chance of dark bow drops seem to be of around 1/300
Strykewyrms - any of these are all good tasks! the reason I choose for this is that they can all drop elite clues aswell as their own unique drop; hexcrest for jungle, focus sight for desert and staff of light for ice, these tasks will definatly give you a decent profit.
Spiritual mages - very easy to kill and have a good chance of dragon boot drops, I average of about 2 boot drops per tasking making it easily profitable.
Metal dragons - although these can be tedious at lower levels when you have decent stats these go by very fast, just the bones are worth 4.5k each and a chance of visage (around 1/10000 though!) and dragon platelegs and plateskirts are also dropped from these, the mithril dragons can drop the dragon full helm fetching prices of over 40m.
Kalphites - these have no drops and are very boring! if you want to do this with a chance of anything the only way is to kill the kalphite queen (level 333) but this will take a very long time
Terror dogs - located in tarns layer which is a spot very hard to access except by using a charge of a ring of slayer, their best drop is the granite helm which is now only worth 40k and they hit constantly and prayer drained quickly in the area; not worth doing.
Skeletal wyverns - they are more powerful than their name suggests! they will use ice attacks stopping you attacking for awhile and hit constantly. they also require a dragonfire or elemental shield to stop them hitting incredibly high antifire shield or superantifire pots will not work here!
Suqahs - No drops!!
Aquanites - these can be unlocked with slayer points to be assigned as a task, do not do it! they attack with magic hitting accuratly even against karils armour and have a attack what disables prayer.