In order of preference:
Rossi is terrific. I liked him best when he was caustic and ploughing his own course as in S3. Ruffled a lot of fan feathers back in the day, all strength to him, and he got under the skin of the more wary characters too -- again, good on you Rossi. He's got a definite 'I am here' quality but you can hardly ever guess what he's thinking. Good job JM.
LOVE Strauss. Dogged, scheming, always personal (in a testy way) and with a lot of human presence despite all that. IMO one of the two or three genuinely strong regular actors on the show. (JM, RN rip, and Jayne Atkinson). There have been of course many talented guest actors.
Reid is the standout character for the half-watchers and for a lot of seasoned watchers too. But he's not a cartoon. They have to be careful with him and Garcia not to make them into jokes. Just a little of the quirk goes a long way and, the more subtle it is, the more you give depth to the world on screen.
Morgan has great energy. Weird how the Morgan episodes aren't as interesting for him as they are for the other characters. Perhaps he's better without the spotlight turned on him. He always come across terrific in the big group episodes and he is the main figure in the CM moment that stands out as most believable and strong (in my book) -- pig farm episode where he nearly runs away from the thing. You would. You're faced with that bleak layout of shoes from the masses of victims... If you don't want to run, maybe you should think of another job for a bit.
Tech girl is fun but needs to rein it in a little, as with Reid.
I have to say I'm not too fond of the others but I did enjoy Seaver last season. She was a proper actor and didn't get a chance from the writers or the fans. Hotch I like better with the beard (hides those lips) but still not keen on him, Prentiss or JJ. They don't seem quite as 'real' as the others somehow but maybe that's just a thing of perspective. They're certainly not unwatchable.