If you can only get one, get the 226E. It has fire box glow, a nice size, and a fabulous 12 wheel die cast tender. It is the ancestor of the postwar 726, 736, 2046, 2056 etc type locomotives. These can be had for around $350-450 in super condition.
The 763 is massive and looking either side of 2 grand for one. I may be wrong, but I think they may need large curves.
The 224 and 225 are smaller and great in their own right. These can be had $150-250 in decent condition.
Again,the 226E is the one I'd go for if I could only have one.