The only variation I plainly don't like is 4...c5. I feel white has a small advantage there he doesn't get in other lines.

I don't care for 4...Nc6 either, for similar reasons, but I do feel it is slightly better than 4...c5.

I think black's best winning try is 4...d5, but I'm not sure he's equalizing in all lines. This is my weapon of choice in tournament play, but I wouldn't take it into a match, if that makes any sense.

4...0-0 is likely best from an objective point of view.

5. a3 Bxc3 6. Qxc3 b6 is the main line that is considered pretty much equal. However, I like 6...d6.

The easiest one to learn is 6...b5!? (I don't know its theoretical status, however)

1. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. Qc2 0-0 5. a3 Bxc3+ 6. Qxc3 b5!? 7. cxb5 c6 8. Bg5 cxb5 9. e3 Bb7 10. Nf3 a6 (eventually 1/2-1/2 in Bareev-Anand, Monte Carlo 2005). Black's play comes very easy, and his control on the light squares grants him an easy equality.

I'm pretty sure the problem with 6...b5 is 8. Qd3!, where an interesting try is 8...Qb6!? If black solves the 8. Qd3 problem then 6...b5 would be a great, active choice.

This is probably on dozens of posts here, but for some reason I can't seem to find it.  I'd like opinions on the best, or most active, answer to the NID Classical 4.Qc2 line.  The Easy Guide to the NID (I think that's the title) recommended 4...00.  I see lots of 4...d5 games, and my teacher recommended 4...c5 and ...Na6, precisely because it's supposed to be "less than perfect" so it might not be subject to the almost hourly theory upgrades and revisions of other lines.

Your opinions are appreciated. 

Thanks!