When and where to use rw,nofail,noatime,discard,defaults?

As said by @ilkkachu, if you take a look at the mount(8) manpage, all your doubts should go away. Quoting the manpages:

-w, --rw, --read-write
   Mount the filesystem read/write. This is the default. A synonym is -o rw.

Means: Not needed at all, since rw is the default, and it is part of the defaults option

nofail Do not report errors for this device if it does not exist.

Means: If the device is not enable after you boot and mount it using fstab, no errors will be reported. You will need to know if a disk can be ignored if not mounted. Pretty useful on usb drivers, but i see no point on using this on a server...

noatime
   Do not update inode access times on this filesystem (e.g., for faster access on the 
   news spool to speed up news servers).

Means: No read operation is a "pure" read operation on filesystems. Even if you only cat file for example, a little write operation will update the last time the inode of this file was accessed. It's pretty useful on some situations(like caching servers), but it can be dangerous if used on sync technologies like Dropbox. I'm no one to judge here what is best for you, if noatime set or ignored...

discard/nodiscard
   Controls whether ext4 should issue discard/TRIM commands to the underlying block device 
   when blocks  are  freed.This  is  useful  for  SSD  devices  and  sparse/thinly
   -provisioned LUNs, but it is off by default until sufficient testing has been done.

Means: TRIM feature from ssds. Take your time to read on this guy, and probe if your ssd support this feature(pretty much all modern ssds suport it). hdparm -I /dev/sdx | grep "TRIM supported" will tell you if trim is supported on your ssd.

As for today, you could achieve better performance and data health by Periodic trimming instead of a continuous trimming on your fstab. There is even a in-kernel device blacklist for continuous trimming since it can cause data corruption due to non-queued operations.

defaults
  Use default options: rw, suid, dev, exec, auto, nouser, and async.

tl;dr: on your question, rw can be removed(defaults already imply rw), nofail is up to you, noatime is up to you, the same way discard is just up to your hardware features.

I came across information that says it is VERY IMPORTANT TO DISABLE DISCARD MOUNT OPTION on SSD's (-o nodiscard) under Linux. Here's the quote and link:

Link:

https://www.intel.com/content/dam/support/us/en/documents/ssdc/data-center-ssds/Intel_Linux_NVMe_Guide_330602-002.pdf

Quote from page 6:

"Filesystem Recommendations

IMPORTANT: Do not discard blocks in filesystem usage.

Be sure to turn off the discard option when making your Linux filesystem. You want to allow the SSD manage blocks and its activity between the NVM (non-volatile memory) and host with more advanced and consistent approaches in the SSD Controller.

Core Filesystems:

• ext4 – the default extended option is not to discard blocks at filesystem make time, retain this, and do not add the “discard” extended option as some information will tell you to do.

• xfs – with mkfs.xfs, add the –K option so that you do not discard blocks.

If you are going to use a software RAID, it is recommended to use a chunk size of 128k as starting point, depending on the workload you are going to run. You must always test your workload."

As you can see, the manufacturer itself, Intel, makes the point SO STRONGLY that they repeat it FIVE times in the text, FOUR explicitly and one through logic that professionals understand. This is VERY EMPHATIC of the manufacturer. Also, this SSD is no slouch: it is the $1,200 Intel SSD's from a few years ago.

I do not know how the "trim" option relates to any of this; I am passing on the information from the document. The document is dated March 2015, Revision 2.0, and is updated to the Linux Kernel 3.19 (the document lists that on Page 2).

defaults, (or ,defaults) is never needed.

I don't know why the example in man fstab uses it :-(.

The reason to use defaults is if you do not need to provide any options. You need some word to put in the options column, if you want to provide value(s) for the next column(s).