all free reward 'Mechs (and one you need to pay for in The Dragon's Gambit)
Campaign - just paint job exclusives save for the Valentina Wolverine II
standard (Freeman's) JR7-D Jenner - Last Man Standing
Razer branded WVR-7H Wolverine II - Hidden Cache Found transmission when travelling to Valentina
Black Inferno WHM-6R Warhammer - First Strike
Black Inferno MAD-3R Marauder - Lock n' Load
Nikolai Mason's VTR-9B Victor - Black Harvest
Kane's KGC-0000 King Crab
Nikolai Mason's NSR-9J Nightstar - Reckoning
High Reward Quests - just stocks
standard LCT-3V Locust - Armed Robbery
standard CDA-2B Cicada - Picking Up the Pieces
standard PNT-9R Panther - Frontline Support Davion
standard SDR-5K Spider - Frontline Support Kurita
standard VND-1R Vindicator - Industrial Espionage
standard ENF-4R Enforcer - Frontline Invasion Liao
standard ON1-VA Orion - The Enemy of My Enemy
standard QKD-4G Quickdraw - Destabilization if you refuse Corporate Insterests path
standard FS9-A Firestarter - Lyran Independence if you refuse The Cost of Loyalty path
standard GRF-1N Griffin - Lyran Rebellion if you refuse The Cost of Freedom path
standard DRG-1N Dragon - A Dragon in Sheep's Clothing
Heroes of the Inner Sphere - Hero 'Mechs, including the only Corsair in MW5 and a Microsoft/X-Box store exclusive
PXH-KB Phoenix Hawk Kobold - Exclusive to DLC purchased via the Microsoft Store on PC or X-Box (unless you use mods to unlock)
DRG-SDW Dragon Sidewinder - Crimson Crusade High Reward Quest
RFL-DNA Rifleman Diana - The Conspirators High Reward Quest
ARC-AGC Archer Agincourt - Bow and Arrow High Reward Quest
ONa-YAJ Orion Yad al-Jauza - Hazing of the Weak High Reward Quest
VTR-BSK Victor Bailisk - Bring Her Home High Reward Quest
COr-PVT Corsair Privateet - only Corsair in the game! - X Mark the Spot High Reward Quest
KGC-CAR King Crab Carapace - Stop the Launch High Reward Quest
Legend of Kestrel Lancers - stock versions but some rare examples
standard CTF-1X Cataphract - Tikonov
standard ZEU-5T Zeus - Tigress First Mission
standard MAD-3D Marauder - Dire Circumstances High Reward Quest
standard WHM-6R Warhammer - Tributes for the Imperator High Reward Quest
standard ON1-M Orion - Operation Onoda High Reward Quest
Call to Arms DLC - stock examples
standard HCT-3F Hatchetman - Those in Need High Reward Quest Second Mission
standard ASN-25 Assassion - Those in Need High Reward Quest Fifth/Final Mission
Rise of Rasalhague - Rasalhague is stocks, Bounty Hunter missions are pristine rares and then his Hero 'Mech
standard CRD-3R Crusader - Kirchbach
standard STK-4N Stalker - Predlitz
standard CP-10-Z Cyclops - Gunzburg Second Mission
standard AS7-D Atlas - Gunzburg Third Mission
standard FS9-A Firestarter - Bounty Hunter Intel Level 3
standard KTO-19b Kintaro - Bounty Hunter Intel Level 5
standard CRD-2R Crusader - Bounty Hunter Intel Level 7
standard HGN-732 Highlander - Bounty Hunter Intel Level 9
MAD-BH Marauder Bounty Hunter - Head Hunting High Reward Quest
The Dragon's Gambit - Otomo versions unique variants and fancy painjobs, free except for the easy to miss Grand Dragon!
Otomo DRG-1G-S Grand Dragon - NOT FREE/purchase in markets at Vega ONLY during the DLC missions on Vega
Otomo CPLT-K2-S Catapult - Seek and Destory
Otomo BLR-1G-S BattleMaster - Shielded Withdrawal
Otomo MAL-MX90-S Mauler - Shattered Lines
Otomo CGR-3K-S Charger - Razing the Enemy
Otomo HTM-26T-S Hatamoto-Chi - DLC Complete message from Michi Noketsuna