Seems to me, which heavy tank you take is totally up to your preference.
For me, Leos are the only way to go. I tried Abrams, but the fact that they do not penetrate the T80U frontally (from max range) was a big nono. Challies are just way too slow (imo NATO is at a big disadvantage when defending) and the AMX32 is way too squishy.
Necrotastic! All the other posts in this thread were before the heavy tank range buff. They were also before the Leo 2A4 accuracy and stabaliser buff. These days the 2A4 stands a clear leader, with the Chally and M1A1 being rather similar to each other.
Those days the Leo 2 only had a very good stabiliser and accuracy 9 on the main gun IIRC. It was also 1925m range, same as all the other tanks, so the weaker side armour was much more significant.