Let's talk about Alchemists and combat applications of poison. They can create it freely at the beginning of the day, handing it out to allies to use. First I'll talk about methods of application and their issues/opportunities, then the poisons available that might look promising.
Methods of Application
1. Contact Put it on an item or "directly on a creature's skin." How does one do the latter? Unarmed attack with a vial? Improvised weapon attack with a vial? Only on a paralyzed, unconscious, or otherwise helpless creature? Probably the latter, so not very useful except by using it to trap items. Not very practical for PCs, I won't be talking about these below.
2. Ingested Apply to food, make them eat or drink it. You could use Suggestion or a similar spell to make someone consume it, but the onsets are always at least one minute, so it's an assassination tool where you somehow flee and wait for them to start dying (hopefully without finding a counteract) in your absence. Not practical for PCs, I won't discuss them.
3. Inhaled Single action activation releases a 10 foot cube that lasts 1 minute, but has a 1 round onset delay after exposure. If you know it's there you can single action hold your breath for a +2 to your save. The obvious move is to have a poison resistant PC open it adjacent to a couple of enemies while holding his breath (and having time to be healed if necessary). The pro move is to have an Unseen Servant do it, 30' per move, one action to open (L bulk, they can, GMs), 2 actions per turn. Muahaha, Paizo, your efforts at totally nerfing this spell have failed!
4. Injury Three actions to apply, requires both hands (one for poison, one holding the weapon). But there's no duration listed for how long it can stay on a blade, so subject to GM discretion you can do some reasonable preparation with an arrow or a blade that is unsheathed. The first P/S damage you do with a Strike applies the poison, and misses retain the poison unless you crit fail. This is the bulk of reasonable PC use.
Poisons
I'll subdivide these by type, and list level regular DC. In a later post I'll look at the DCs as the compare to optimized class DCs of the Alchemist at that level. Note that with the 10th level feat Potent Poisoner an Alchemist can apply a +4 to base DC.
If you want the effects of individual poisons look them up. I'm assuming that if you can make a few at the beginning of the day with decent DCs you'll find some that are worth your Alchemist's reagents.
Inhaled Poisons
(Not much here, and all high level.)
Level 15: Mindfog Mist, DC 35
Level 16: Brimstone Fumes, DC 36
Level 16: Nightmare Vapor, DC 36
Injury Poisons
(Plenty of progression at low levels, but skips some levels in the middle and then stops.)
Level 1: Giant Centipede Venom, DC 17,
Level 2: Black Adder Venom, DC 18
Level 2: (Uncommon) Lethargy Poison, DC 18
Level 3: Cytellish Oil, DC 19
Level 3: Graveroot, DC 19
Level 5: Hunting Spider Venom, DC 21
Level 6: Giant Scorpion Venom, DC 22
Level 7: Giant Wasp Venom, DC 25
Level 10: Shadow Essence, DC 29
Level 13: Purple Worm Venom, DC 32