Signal is the easiest alternative for me when I try to make friends transition to a better messaging app. It's pretty lightweight to install, secure (there are potential security issues but only the most hardcore tech enthusiasts care about them, overall it's miles more secure than common alternatives. Check on twitter if you really want to learn about it), has nice features (delayed response, stickers ✨), and the onboarding is great.
Telegram I never used, has a reputation of being used worldwide for illegal communities (drug dealing, scammers), which is dumb because it's one of the least secure messaging apps out there. I advise to never use it.
Small aside, I hate that whenever I join a club, work in any event, or do anything in a group of normies,everyone turns to WhatsApp group. This is especially egregious when you're for example doing volunteering work and need some documents or knowledge. "It's in the WhatsApp group!" Oh great, sure, add me. You join the group and there's no history (for you, don't worry Meta servers will keep all your messages safely for their own purposes), so you have to tell them to send the documents again anyway. You could refuse to join the spyware, but in these kind of groups it's hardly an option unless you have a good friend already in the chat that will relay the relevant information to you. But if you're good friends you'll probably talk him out of using WhatsApp...
What I like to use is matrix, but good luck getting people unfamillar with tech on it.
On paper it's great, it's an open source protocol (like IRC), highly secure, tons of features for rich messaging, decentralized servers, and tons of stuff I forget. But in practice the onboarding is horrendous, available clients are a bit clunky, messages can take very long to load especially in encrypted discussions, the search feature doesn't work well, sometimes the client breaks...
All in all, I really love it and I encourage you to try it, but I couldn't force people that aren't slightly into tech to transition to it.