This is my favorite of all of the Red Kryptonite episodes. Smallvile established the rule that Red K takes away Clark's inhibitions, which several episodes have gone too far with. "Crimson" sticks closer to the rule that Clark would act on and say things that would already be on his mind, even if out character. Season 3 I think went too over the top, with Clark stealing and breaking laws, which I always felt was too much of a stretch for a future Superman. I'd rather see Clark acting like a jerk than as a flat out villain, which several Red K episodes did to him in the past. This time the worst that Clark does on Red K is insult and call out all of the other characters, Lex, Lana, Martha and Chloe, on things that as an audience member you've already questioned. Even though Clark is going out of his way to be as confrontational and hurtful as possible, he doesn't say anything that's untrue. Again, unlike how season 3 handled Red K episodes, "Crimson" doesn't get too dark, but instead keeps the story flowing in a fun, entertaining and sometimes humorous way.
While most Smallville seasons start off very strong and slow down considerably in the 2nd half, season 6 does the opposite. The first 9 episodes were mostly throw-aways, and Crimson marks the 4th straight classic episode in my opinion. Up until this point I had felt that the Lex/Lana/Clark love triangle was most interesting than most have felt, although I can admit that too little was happening. By the end of Crimson the story progression is really sped up, and actually left me quite interested in seeing how it continued.