Recently, M1 offers a $5 plan for senior citizens. Is this the best plan for mobile available. Please advise. Thanks

If your senior mainly just needs basic calls, SMS and a bit of data then the $5 M1 plan is quite hard to beat for value, but if they use more data or have other needs it's worth comparing with the other telcos before deciding.

I usually prefer singtel plan due to the stability of the network and reasonable customer support. Price is not the main priority.