AWS, nginx and LetsEncrypt
AWS certificate manager integrates nicely with elastic load balancer.
If you plan on using ELB then ACM would definitely be the way to go (if ACM is supported in your region) because certificates will be managed by AWS.
If you plan to stick with Let's Encrypt you will likely need to use Nginx for your frontend so that you can easily configure auto renewals there.
You can find more information here:
https://aws.amazon.com/premiumsupport/knowledge-center/associate-acm-certificate-alb-nlb/