If you meet someone using iMessage or Google Messages, I’d consider encrypted RCS via Google Messages. Otherwise the next best “mainstream” options are between WhatsApp and Telegram. WhatsApp’s E2EE has issues but Telegram’s E2EE is still pretty bad in comparison. However, Telegram seems to have a better privacy policy than WhatsApp.
If you’re more concerned with protecting the contents of your messages, I’d prefer encrypted RCS > WhatsApp > Telegram. If you’re more concerned about hiding your phone number or handing over your metadata to the likes of Meta and telecom providers, I’d prefer Telegram > encrypted RCS > WhatsApp. In my experience, finding a Telegram user IRL is as rare as finding a Signal user. If Telegram isn’t super common where you are, you may be stuck with WhatsApp or encrypted RCS regardless of what you deem a lesser evil.