You have about 5 Main Choices, with Grey Areas between each
All builds will generally start with a heavy base of at least 2 Psyrifle Dreads, if not more. Best use of 135 points in a
GK army...
HQ is a mix of personal preferences and tactics like anything.
Draigo, Coteaz and Crowe are requirements for three of the builds...after that Grandmaster is a solid tactical choice and the beginning of a Deathstar unit, but expensive at ~200 points with Rad and Psychotroke 'Nades at the minimum.
Then comes the Brother Captain (
GM lite), the 25 points for Grand Strategy is well worth it if you want a Terminator
GK HQ.
Brotherhood Champion is eh, same as Crowe basically, for 50 less points and lacking the redeeming factor of purifier troops. Not a popular choice and for good reason.
Librarians: Can be your Swiss army knife, but better if you pick a role and gear them out towards that end.
Inquisitors...Cheap and can be geared for lots of different jobs. Most popular is grenade bearer for an assault squad. Rad+Psychotroke given to a squad for less than 100 points.
Anyway...
1. Power Armor Grey Knights: Strike Squads at the base of the army, using Purifiers in Elites to Cause damage in
CC as well as the additional fire support of 2 psycannons per 5 purifiers.
2. Purifiers: Crowe+Purifiers+Psyriflemen Dreads with some spices tossed in of your own choice. I personally run mine with a dreadknight or two, but exact lists are to taste and playstyle.
3. Paladins/Terminators: Terminators...they are exactly what they've always been... 2+/5++ beastlies that do damage but cost premium. Paladins cost even more but are worth the extra 15 points per. You need to find a way to move them around the table. Stormravens, Deep Strike or Land Raider mobility. Draigo is needed for Pally army and comes with Grandmaster toys, paired with a librarian with shrouding and storm ravens you have a dangerous assault army.
4. Inquisitor Warband: Coteaz takes charge of this crazy crew giving you henchmen as troops. Death Cult Assassins and Crusaders drive the
CC side of the army and Jokaero, Acolytes, Servitors drive the Ranged Support. Chimera good times.
5. Hybrids: Coteaz Henchmen+
PAGK, Purifier+Henchmen,
PA+Term, Pally/Purifier, etc. Lots of options once you leave the themes. Really need to think of a direction before you buy models as theres a lot of choosing to be done.