The metro in Paris had an objective of more than 96.5% of reliability in 2018 according to https://actu.fr/ile-de-france/paris_75056/du-rer-b-ligne-13-... (in French)

All lines fulfilled this objective except line 4, with 96.1%

AS for RER, there is an official governemental website that gives the numbers: http://www.qualitetransports.gouv.fr/les-chiffres-de-la-ponc... (in French) The most reliable one in 2020 is the A with about 94% of reliability and the worst one was RER B in March, with 78.9%.