[Patrick]

The 2.5d (1650kv) is good on 5 and 6s. It's ok on 4s, but depending what your track is like it might not have quite enough speed.
1y (2200kv) is good on 4s, and pretty fast for the track on 5s. If your track isn't that big it might be a bit too much on 5s.
There's also the 2D (2050kv) which will be quite similar to the 1y on 4s after a slight gearing change, and might not be as much over kill on 5s if your track doesn't need heaps of power. Could be a happy medium motor.

All three motors will work and won't be too far off each other if you adjust gearing, but which one is best for you just depends on what sort of driving your doing and what you expect.