Shimano's HF08 rear tandem hubs are probably the most durable, best value and use a steel body.
White Industries use steel for tandems and offer an optional titanium body.
Phil Wood uses steel for its tandem hub bodies and axles,
Chris King also uses steel bodies and axles.
Aluminum and hard-anno aluminum will deform unless you use a cassette where at least the 5 largest sprockets sit on a carrier.