The foundational 26-episode story. Final two episodes are famously abstract due to budget constraints — expect this going in.
The film that delivers the conventional plot resolution the TV series couldn't. Watch immediately after the series, not as optional.
A separate continuity that retells then diverges from the original. Complete on its own — not required to understand the original series.
No — Rebuild is designed as its own complete, separate continuity, though watching the original first provides useful context for how much it diverges by the third film.
The final two TV episodes shifted to an abstract, introspective format due to budget constraints late in production, leaving the plot's conventional resolution unaddressed until The End of Evangelion film.