I've only played a few games of 9th (2 against Ravenwing/Deathwing, 2 against Thousand Sons and one against Tyranids). I find Linebreaker or Engage is a good bet, and I could see myself taking it in most situations. Between Interceptors, GoI and general Deep Striking ability those secondaries should always be attainable. The choice comes down to deployment zones and your opponent's ability to screen you out' IMO.
Attritiin can be a good one I find, as many Grey Knight lists tend to focus around on a few really tanky TEQ units, with any power-armored support either protected by deep-strike or terrain (e.g. Purgatory, Strikes). Most opponents will focus on your Paladin/Terminator bombs or your GMDKs, meaning against most factions you should be able to score more unit kills. So far this has been true in my experience anyway. An alternative against horde armies with large units of infantry would be Thin Their Ranks, betwixt Storm Bolters and Psilencers, hordes should rarely be an issue.
Assassination is a good one, our characters can go toe-to-toe with most other faction's characters, so they can be used to score those points. Bring it Down can be a tricky one, as GK
do tend to struggle with vehicles, but if you're packing the right firepower and you're facing a mechanized force, then it's definitely worth a punt.
I'd personally avoid secondaries that require actions, as stated already we have so few units on the board as it is, and really we need each of them doing something in each phase to get the most out of them. However, Reskin's idea of taking the one character (in his example an Apothecary) who's already kind of limited in what he can do in each phase, and who completing an action won't cancel out any special rules, to score Psychic Ritual is a pretty sure way of bagging those points. I'll have to give it a shot myself. Could a cheapo Inquisitor be used to similar effect? Doubling up on the CP
Regen power (Mental Interrogation I believe) and using him as backline points scorer that isn't costing you any of your GK
powers could be a solid tactic.
Overall though, it really comes down to what mission you're playing, and against whom. Does the mission-specific secondary lend itself to being an easy way of scoring points? Does the deployment favor Engage or Linebreaker? What unit types is your opponent fielding and how can you capitalize on that to score points? Do you have/is it worth "sacrificing" a unit to complete an action each turn? You get the gist. Just my 2 cents, good hunting to you, lads.