I agree with AA/UA are the best choices unless the OP would be buying business class tickets for Europe than CO may have a slight edge by the frequent Z class sales and AF.

If between AA/UA, I would say AA due to 3 things:
1. Challenge so you can build up status fast.
2. 1 MM / 2 MM includes credit card miels (for now) so the user can transfer his MR to increase total quickly and potentially even direct business spending there.
3. The upgrades are less expansive to purchase. UA has more upgrade instruments for top tier (SWU, CR1, 500 milers) but AA stickers are less expansive if the OP might be buying cheaper fares and upgrading.

Have done AA JFK-CDG and it was one of my best transatlantic in terms of service but I had an awesome crew on the way back. LH seats was slightly better than the AA seats but the champagne was definitely much nicer on AA.