mouseno4: The season pass isnt available?
It is:
https://www.gog.com/game/battletech_season_pass but you can get the Mercenary Edition, which has base game and season pass included.
Flashpoint adds the most "story" content, great easpeically if you read the novels in former times and know te lore a bit. Alot of points to laugh and chuckle.
Urban Warfares biggest addition is ECM, so electronic warfare. Was more like cheating in the beginning, because you became invincible if played right. Still makes the game way easier now. And the Urban biomes although quite laggy are very nice to pla (shoot at buildings to offer you something to hide *cough* very ... errr ... merenary like solution ;) )
Heavy Metal for me personally is the worst DLC (which I still like very much), it adds alot and great Mechs with unique abilities and alot of new weapons which make fights a bit more different, buuuuuuut rare weapons arent rare anymore and the game gets really really easy. If you like the lore, those weapons and eq were very very rare. All of my mechs now have Double Heatsinks, even the lights and since release of UW I am mostly in the outsides of the map to finish my mission to visit every single planet and doing every single mission there.
On one planet got 4 parts of an Atlas II, there was one complete Atlas II I didnt buy and several Snubs and UACs and a double HS, just tooo much extremely good rare eq in one place.