Depends on what radii your are running.
But knowing what I know now.
Diesels: really a TMCC engine from 2003 or later are great. It will run great with Legacy too.
Steam: Bang for the buck I would get a Berkshire ; 765, 1225, or some of the newer ones being released with whistle steam. If you are buying an engine right away, look for specials, right now you can see some of the big 2-8-8-2 engines are on sale.