Some of the basics are 1 special weapon per 5 models in the unit. Meaning two psycannons for a 10 unit are ideal.
Psiliencer: Are heavy 24" range weapons. Making them less than ideal for anything but a terminator who can move and fire them.
Incinerator: I am not a fan of these flamers in strike squads. Either you make them shooty, or close combat oriented (which means halberds and you do not want to kill too many or make a unit fall back if you are going to assault).
Psycanon: We all know this one. S7 Ap4 rending. 24" range, assault 2 or heavy 4.
The Psycannon really is the only choice for a strike squad. They are relatively cheap here (10pts), so are the best special weapon to use. If you are not going with a special weapon for some reason, the other option really comes down to all close combat weapon upgrades.
Deepstriking:
This is in my opinion where the strike squad excels. The simply come with the ability to deepstrike. Its a rare ability to just have this as an option for your basic troop choice.
One of the interesting things with Grey Knights is how they can manipulate reserves and deepstriking rolls. Many of your HQ choices come with psychic communion. If you are using Grey Knight Terminators or Strike Squads, manipulating reserves is as simple as taking two HQs with psychic communion, where you get to adjust the reserve rolls after you roll the dice by up to +2 or -2. This gives great control on deepstrikers when they arrive.
Servo-skulls reduce your scatter when you arrive. It is true that they can be removed, but if that is taken into account it becomes an asset, as it is one more thing your opponent is quickly trying to do. With Deepstrikers sitting in reserves, it becomes a tactical asset. Servo-skulls reduce your scatter by one die, so you are much more accurate.
Psybolt Ammunition
In no other unit are the points spent for psybolt ammunition so cost effective. For a 10 man strike squad with psybolt ammunition you are effecting 8 models with storm bolters, taking them up to S5. With this idea, it is quite possible to field an entire army with no or very little S4 weaponry.
Strike Squads are the on place I enjoy seeing psybolt ammunition used. Its very complimentary to having two psycannons, and when shooting at armour value 11, it makes sure that if your psycannons do not destroy it, you should get 1-3 glances from your storm bolters to render the vehicle inoperable.
With the flexibility of Deepstrikers being able to combat squad into different locations when deepstriking, and the manipulation of your reserves and deepstrike rolls, Strike squads suddenly become a very tactical asset on the table top. When they appear, they should be able to tear something up with psycannons and storm bolters, and they can appear wherever and whenever you choose.
Warp Quake
The tactical flexibility advantage of the Strike Squads is something often taken for granted. Add in Warp Quake to that list, as daemon players and others with strong deep striking abilities will attest to. Warp Quake provides a deep striking bubble of 12" surrounding the unit. This can literally effect a large area on the board with only a couple combat squadded units doing it. With their upgraded shooting, this allows them to counter punch anything that has to come in and land outside of where they were intending.
Deepstrikers are often meant to be a punch in the face to your opponent. Warp Quake quickly takes that away without much effort, and puts opposing units within optimal range of your guns. Just sitting there for you to take out.
Conclusion:
The Strike Squad was not my favorite unit of choice when the codex came out, nor for a long time. However they are such a forgotten asset in the Grey Knight codex, that they deserve more playtime. In the world of Draigo Wings, Coteaz, and Purifiers, the standard troop unit of the codex is not quite so shiney and most often left out of Grey Knight lists.
My current Strike Squads are 10 man units, given psybolt ammuntion and two psycannons. Total cost 240. Their objective is to drop into the fray at a time of my choosing and shoot the crap out of things. I do not upgrade their basic close combat weapons, simply for cost effiency, and the fact that having a power weapon as your basic weapon is good enough for me.