Dual 4870's really aren't the best anymore. They are really good if your on a budget though. But the OP did say the best, and well the 4870 isn't the best anymore. If I had to choose then I'd go with a Nvidia GTX 295 or a Radeon 4890
You do know that the 4870 X2 is a single card with 2 gpus?
For most benchmarks, the 4870 X2 comes in second for the most powerful single video card.
This is the normal pecking order for high resolution and high rendering in games:
1. NV GTX 295
2. ATI HD 4870 X2
3. ATI HD 4850 X2
4. NV GTX 285
5. NV GTX 280
6. NV GTX 275
7. NV 9800 GX2
8. (tie) ATI HD 4890
8. (tie) NV GTX 260 C216
10. ATI HD 4870 1GB
11. ATI HD 4870 (512MB)
This is not absolutely cut in stone. Some benchmarks show the 4890 bettering the GTX 275
Some bench tests show the 4870 1GB being more powerful than the GTX 260.