To give you a simple answer - Panda Gold Supremes. Good quality, affordable, usually grow up to be gorgeous fish and generally come in at a good size to start a community.
However, to each their own really. All my high end fish are/will be from farms such as pang long and xien leng, but for I used panda fish when I did an arowana community and I was happy, so I will probably go back to that.
I just think an rtg community looks just as good as a xback community. Especially considering xbacks in communities generally don't meet their full potential. I always found it harder to focus on the quality, health and care of the fish in a community tank than in a solo tank. Plus the quality of an expensive fish diminishes quick when scales are being picked off and fins are being torn, which happens often in communities, so why waste the money.
I don't see why someone would spend all that money for xbacks when an rtg community looks just as impressive. IMO High end arowanas are best raised alone to focus on the full potential of the fish and RTGs, or lower quality xbacks/reds, are best for communities. Unless you have a butt load of money to waste.
But like I said to each their own.