Please explain me the use of "Reselect on Service Down" and "Least Connection" and also which is better? Consider me new to this

Just including the following ref https://support.f5.com/kb/en-us/solutions/public/15000/000/sol15095.html, to warn on the Reslecet option - it is NOT what the documentation has traditionally said - only works in very specific cases

This option is only appropriate for:

* Virtual servers with address and port translation disabled
  *Note*: This is default for FastL4 type virtual servers, such as network or wildcard forwarding.
* Transparent pool members, such as firewalls, routers, proxy servers, and cache servers
  *Note:* Transparent devices can forward packets to destinations without regard for the state of the connection.
UDP virtual servers

"None" is the default for good reason.