Gotta go with Suchet, then Finney then Ustinov.

But far and away the best acting for a repeated Christie protagonist is by Joan Hickson for the Miss Marple tv series. She understands not only the character herself but what Christie is getting at with the characterization. I would also argue t hat as a literary character, Marple is superior to Poirot.

The Poirot novel "Halloween Party" features a female detective novelist who expresses regret for having created a Finnish vegetarian detective without knowing anything about Finland. Christie occasionally dips into metafiction, but this is probably her most amusing example. Christie had no problem with an elderly British woman whose wisdom is often ignored by more rationalistic male characters.

FWIW, Ackroyd is the only Christie mystery I haven't read, because some high falutin' literary critic gave the twist away. (Literary critics seem to feel they're above giving spoiler alerts.)

The most interesting Christie novel for concept and execution (and the best twist after Ackroyd) is Towards Zero. There's a post-Hickson Marple adaptation but I haven't seen it; the original doesn't feature a recurring detective. Towards Zero ends with the ultimate crime rather than starting with it, and uses a third person narrator to get into the perpetrator's head without giving away the perp's identity.