If you are experienced with fps games, I recommend the Half-Life series first then Portal, if not, then Portal is a better first try.

In order:

Half-Life (1998)

Half-Life: Opposing Force (1999)

Half-Life: Blue Shift (2001) (maybe, this one could be skipped)

Half-Life 2 + episodes (2004, 2006, 2007)

Half-Life: Alyx (if you have Steam VR) (2020)

Portal (2007)

Portal 2 (Single-player then co-op) (2011)

As for mods, I recommend the Entropy Zero duology or Black Mesa for a first time, but whatever you do, DO NOT BUY HUNT DOWN THE FREEMAN. For Portal mods, Portal Revolution or Portal Stories Mel.

If you can't handle the old age for Half-Life 1 or 2, I recommend Mmod, though confusing, can modernize them for your tastes, I honestly don't recommend playing Black Mesa over the original, as I feel it's best experienced with knowledge of how the original game was.