The best starting material for the preparation of oxygen through heating trioxonitrate(V) (nitrates) would involve a metal that facilitates the decomposition of the nitrate to produce oxygen, coincidentally, Pb (Lead) falls into that category.

Lead(II) nitrate (Pb(NO\(_3\))\(_2\)) decomposes upon heating to produce oxygen, nitrogen dioxide, and lead(II) oxide. This reaction is often used in laboratory settings to generate oxygen.

  2Pb(NO\(_3\))\(_2\) → 2PbO + 4NO\(_2\) + O\(_2\)

This makes lead (Pb) a suitable choice for generating oxygen from nitrates.