I've been doing a lot of slayer recently and it's allowed me to gather information about certain tasks and if they're even worth doing or you should just skip them.
Note: This isn't a guide on the best/fastest way to 99 slayer it's just the fastest way I've noticed to gather points. All information was gathered from an account with 126 combat and therefore some tasks may deviate away from the initial task list, if you're not around the same combat level.
I'll also be referencing numbers next to each task you can receive giving my personal opinion on the indicated amount you should do varying from 8-23 if you're strictly going for slayer points.
Have you considered listing them by how efficient they are for accumulating slayer points? That'd make it easier to tell which tasks to do and which ones to skip for the highest amount of points per hour.
EDIT: Changed into order relative to the efficiency of the task.
Bloodvelds - (8-23) Whenever I get a Bloodvelds task I complete it despite the amount, they're a low combat level and are quick to kill as well as there being a few so you don't have downtime between spawns.
Dagganoths - (8-23) In my opinion the second fastest time to kill ranking just after Bloodvelds, their relatively fast time to kill and the fact there is a large group of them means they're a very quick task regardless on the amount you have and they're a 100% complete task if you get them. As a side note I'd recommend that you kill them closer to the ladder to Dagganoth Kings as 4 spawn there.
Dust Devil - (8-23) At 93 combat Dust Devils are an absolute walk in the park, meaning whatever the amount you receive you should definitely complete. Much like the other three 23 kill tasks they're very quick to kill and there are many of them within the vicinity of one another.
Fire Giants - (8-23) Fire Giants can be quite a pain to kill due to the Wild Dogs however, using this spot http://imgur.com/DYyEEGR will allow you to avoid the dogs if you follow a rotation behind the rock, the spawn time is very quick for Fire Giants meaning that although you're killing two, one of them will always be spawned.
Green Dragons - (8-20) You'll be using the wilderness for this task which changes my opinion on how many you should do. To get the Green Dragons you first want to find PK Teleports in your spellbook then go to Dragons Multi and run south-west. (PK Teleports > Dragons Multi > Run SW) They're very easy to kill but considering you're in the wilderness you probably want to get out as quick as possible, completing a task above 20 kills is optional.
Gargoyles - (8-18) Gargoyles are pretty similar to Blue Dragons with their time to kill meaning I'd recommend that you do any task which is between 8-18, any task that goes above this amount I'd probably just cancel for an easier task.
Blue Dragons - (8-18) I actually feel like Blue Dragons are one of the better tasks you can get as a hard task, they're 111 combat and therefore die relatively quickly if you're just using a whip, therefore I don't tend to skip if they're between 8 and 18.
Hellhounds - (8-16) Although Hellhounds are easy to kill, them having a 122 combat level increases their defensive stats aswell as their hitpoints, meaning this task isn't as quick as others and above 16 kills I'd say you were wasting your time and it's probably better to get an easier task.
Black Demons - (8-16) Black Demons are a really nice task to do, they're not very time consuming and they don't really hit anything. I'll usually do them if the task is 14-16 and they're not populated, if they're populated however I'd change tasks, unless your amount is lower.
Nechryaels - (8-16) Nechryaels are much like Hellhounds, they are easy to kill and require minimal effort, however because there are only two at ::train and they are a high combat level their hitpoints are high, meaning it's probably not time effective for you to kill above 16 unless you are at the Slayer Tower then I'd recommend a maximum of 18.
Abyssal Demons - (8-16) I usually skip any Abyssal Demon task that goes over the 14-16 mark because I feel like it isn't time efficient for you to continue killing them and you could have done an easy task and another hard task in the meantime combining for 8 points instead of 6 and having invested a similar amount of time.
Black Dragons - (8-12) Black Dragons are usually populated and therefore it's personal preference if you even want to complete a Black Dragons task. This in cohesion with them being quite painful to kill in comparison to other hard tasks means I'd cancel on any task above 12 kills as you have a relatively high chance of receiving a better task after you've completed an easy task.
Dark Beasts - (8-10) Dark Beasts are on par with two other tasks in my opinion and sometimes I don't even feel like killing 8 of them, this task is very slow and perhaps even attempting this task is bad in terms of time, but I had to acknowledge them. The choice is yours if you want to even bother with this task but anywhere above 10 and I'd 100% skip.
Iron Dragons - (8-10) Iron Dragons are on par with Dark Beasts from my experience, their high defensive stats and the fact you have to run across the room between kills means you're experiencing downtime between kills as well as them taking a long time to kill. I skip these but it is personal preference, anything above 10 should be an instant skip.
Steel Dragons - (8-8) Steel Dragons are basically a more annoying version of Iron Dragons and as soon as I see the word given to me as a slayer task I skip. Their combat level means they have super annoying defensive stats and high hitpoints all contributed to by the downtime between kills. Like Iron Dragons they are personal preference and I'd kill a maximum of 8.
As a quick tip, any slayer task that can be completed on the third floor of the Slayer Tower (Nechryaels, Abbysal Demons, Dark Beasts and Gargoyles) should be, once you teleport to the Slayer Tower you can double click the stairs blocked by the wall and if you click twice quick enough you'll be teleported straight to the third floor, decreasing your down time.
Slayer helmet has no effect on the amount of points you'll receive from each task, thanks Kuji!
Thanks for reading, any feedback is appreciated.
Guide Written By Lev!