The short answer is "none of them" because poisons are horribly overpriced and woefully ineffectual due to awful DCs but that's not the most helpful of responses.
Gun to my head, I'd probably say you get the most bang for your buck via Drow Poison. 75 GP is about as low as the price tag gets and the effect actually can get some work done at lower levels before the DC regulates it to a joke.
The CHEESE answer is use drugs instead. They are super cheap, hard to beat the DC's consistently and lasting. Most GM's would probably quickly have them confiscated at the city gate though for their clearly Munchkin use.
Drow poison is fantastic, sleep is very powerful in a fight.
The ability to use a racial poison is the only real cheap way to get a scaling poison (Grippli, and Vishkanya), so hopefully you can convince a party member to help you there. Some GM's allow for the collection of venoms from familiars or pets, though these do not scale. I am fine if players want to try this.
Otherwise it comes down to spending lots for whatever you try to and stacking doses for scaling DC's. (Or using alternate poison rules for poisons that work 100x better).
All that being said, small centipede poison is great for just being annoying. 1dex damage for 4rnds 1 save at DC11 (90gp/dose). Even when it hits it isn't likely to even drop defenses at all. But man, do players hate rolling against an unknown poison DC. Plus, if they get it as loot it is either use a DC11 poison, or find some way to sell something illegal =D
Black Adder Venom
120 GP, DC 11, 1/round for 6 rounds, 1d2 Con damage
This is the most cost efficient constitution damage you can buy, with the step up to 1d3 increasing by nearly 4x the cost. Stacking doses of this is the best way to kill a low con creature in no time flat.
100 GP, DC 12, 1/round for 4 rounds, 1 Con and 1 Wis damage
A surefire Cleric killer, stacking this up will make up for the lacking duration as well as making the target vulnerable for both fortitude and will save failure. This is also the fastest mental stat damaging poison, as the rest are all 1/min frequency, which really is a shame. Sadly it does have a one round delay, but one round isn't terrible for something this nasty.
75 GP, DC 13, 1/minute for 2 minutes, Unconscious for 1 minute/Unconscious for 2d4 hours
And the knockout blow. No onset, instant sleep, and the chance to put them out for hours is just too good. This is probably the bread and butter of your poisoning set, making it easy to knock out anything that breathes.
Medium Spider Venom
150 GP, DC 14, 1/round for 4 rounds, 1d2 Str damage
The cheapest strength damage we can get, this loves the duration stacking from adding multiple doses, and has a good chance of knocking out brutes if you can get it early and stay away from them.
Stolen from: https://docs.google.com/document/d/1hChbcEsEfQsR7NkwKlzO-GLYtrOtxlkGHpRQgKK Z5gc/edit#