With angels my personal opinion (just an opinion - not being authoritative here) is that one will do best if they purchase those which are as close to the wild type as possible. Those would be the classic black barred silver angels. Especially those with red or as much red in the eyes as possible. That is usually an indication that they are not "junk" angels which are so terribly overbred for the aquarium trade that they have all but lost their natural instincts and are not very hardy at all.
Next the tank should ideally be a classic South American "blackwater" type of environment; much like the type preferred for discus. Relatively slow water movement, lots of driftwood and plants, somewhat subdued lighting, and a soft, acidic chemistry that is achieved either by the driftwood, or with peat filtration. (I personally like and use both at the same time.) Of course all of that specialization isn't entirely necessary or sometimes not very practical (for example it's somewhat difficult to have a highly planted tank with subdued lighting) but if you truly want the absolute best results with angels, that is the way to go.
Without all the specialized action, it's as the others have said. A steady pH that is acid to neutral, clean water, good tankmates, proper maintenance, etc. All of the things that any good fish deserve.
Glad to see someone is doing their homework before plopping a new species of fish into their tank. I wish everyone did that.