BELT PETS
The stats aren't all you need to think of when considering filling your belt, if its 4 slots or 6. You also need to think of what you will be doing and making the most out of your stats gained from belt. EG - You aren't going to tank, so why put stats towards P.Def and Vit?
Try and aim for enough benefit from your belt to hit 349 attack speed. This will give more benefit due to Assassin Impact and Phatasm.
Slayer can make use of many belt sets. Only one or two are exactly what you need though. I was going to go through a list. But in reality a lot of them will just be useless in the long run and people might go for them thinking they are really good and stick with them.
With Slayer you will need a few belt pets so swap and change quite a lot. But the base will be virtually the same.
The cheaper option is Kent (s0) - Kent (s0) - Skel (s3) - Harpy (s3) - Uni (S3) - Boss Card
This offers 32% Str/P.Atk - Dex 16% - Atk.Spd 32% - Evasion 16%
You can increase the Skel / Harpy / Unicorn to help with Dex and Evasion. In HM and Cube you will need dex, in cube you will need it until about 160 in your belt. Maybe P.Acc, I am still to test this as I was 158 when I started soloing cube.
The most expensive belt but the best one I was able to work out, and it's the one I use.
Death Tyrant (s1) - Death Tyrant (s1) - Death Tyrant (s1) - Unicorn (s2) - Unicorn (s2) - Boss Card
This offers 33% Str/P.Atk/Atk.Spd - 32% Agility/Evasion
Whilst I say this is the best build I have come across, this is not reccomended for low level Slayers. You have no dexterity and p.acc and so will miss a lot of attacks! However, you can swap a unicorn or your boss card for a Nightmare.