1. Guard - still the king of the hill. With a huge volume of fire, long range, and their mobile bunkers they can keep grey knights at arm's length. The weakness of GK is that they lack range other than the riflemen dreadnoughts. They die to shooting like any MEQ does. Guard don't care about the hand-to-hand abilities of GK. They lose hth engagements regardless.
2. Grey Knights - after a bit of playtesting, these guys really are pretty amazing. The mech MEQ that is par for the course is even tougher with them, and I think the tourney scene will see a few purifier units in rhinos with psycannons as standard. Add in Cochise for cheap plasma cannons and troops and riflemen dreads, and it will be a tough nut to crack. Grey knights trump other marine armies in hth due to halberd, hammer hand, and power weapons. In fact they don't trump them - they eat them alive. They perform mech as well as most other marine armies, but are much more offensive in hth but not any more survivable.
3. Blood angels - fast shooty mech will still hold its own, but with psyker fortitude on their vehicles and str 8 rending psycannons from hatches, grey knights will usually come out on top of the short ranged mech shootout. Blood angels will be faster and will have to use that to their advantage to engage targets only after they are weakened with shooting. Blood angels vehicle speed moves them up one spot against GK.
4. Space wolves - Thunder wolf cavalry is brutal and should always be taken you say? Not any more after force weapons. However, Those long fang missile launchers just increased in value though, and so did the wolf scouts coming in off the back edge to eat the GK squishys guarding their objective. Grey knights drop a spot to 4 - grey knights win assaults against grey hunters
5. Dark Eldar - super fast and long ranged - should fare pretty well against grey knights. If grey knights become very common, dark eldar become a slightly more powerful tier as a result. For Dark Eldar facing grey knights will largely be just like facing any other marine army - the only real difference you care about will be halberds allowing them to swing simultaneously as you.
6. Nids - urgh! Not good. All those MC's are in real danger to all those force weapons, and thats if they survive the initial psycannon spam. That bubble wrap is the perfect fodder for cleansing flame. Nids fall a place as their viability takes yet another hit with the newest dex. They may need to be one or two more slots down.
7. Tau - may actually move up in viability if grey knights become popular on tables. High strength ranged fire will kill gk just as fast as the usual MeQ and the hit and jump crisis suits keep the relatively slow GK at arms length. This is a standard marines fight for Tau.
8&9. Chaos Marines and SpaceMarines - Im sorry. The trend continues. Everything you can do, other marines do better. With GK, the viability of both lists is hurt. I love chaos marines and cant wait to see them in in early 6tedition. If you are fielding blue marines - why? The only reason i can think of is vulkan or shrike hammertime.
10. Eldar - Grey Knights are a tougher scrap than standard marines for eldar due to aegis. But The dificulty in targetting the grey knights is more than made up for though when runes of warding starts killing off guys in all the gk squads. Eldar remain where they were.
11. Orks - really starting to show their age, orks have a tough time with the pie grey knights are serving. Nob bikers take another hit, and mob orks suffer like nids do. Purifiers cleansing flame thins the herd nicely, and psycannons murder orkified vehicles.
12. Daemons - these fall about four places. I didn't belive it until I played a GK list against a tournament caliber fiends/horrors list. Yeah preferred enemy is bad, and daemon bane is another poke, but the real game changer is psykout genades. Daemons striking at initiative one. If you can imagine a whole unit of 6 fiends dying to assaulting purifiers you are spot on, and aren't actually imagining anything.
13. Necrons move down to the worst army in the game. Oh wait they were there already? Necrons shake stun vehicles, which GK ignore. Necrons dont get their saves against GK force weapons. The weak got quite weaker.