I haven't played them all, the first MechWarrior game came out when I was 5, and there must be others that I never played, but I played from MechWarrior 2-4 with all expansions, MWO and MechCommander 2.
From all those, my heart is torn between MechWarrior 2: Mercenaries and MechWarrior 4: Mercenaries.
The original MW2 was too strange for me, I had no idea of who were those "clans" why they were fighting nor the trials nor anything. I thought there were only two. Took me years to know that there were more. There was too much text that I had no idea how to interpret with my 201 level of English. But MW2: Merc was clear and also you had to administrate the contracts, watch out your finances but there was this thing with the diary that made me feel like a real mechwarrior, this included reflexions about the missions, it fleshed out some personality for the protagonist, the originality of missions to escape the Clans or participate in a battles like the liberation of Prosperpina or the defense in Luthien that made me understand more about what the whole BattleTech universe was about. Those were weeks or months-long campaigns where you had to imagine that you were paid to do uneventful patrols and move around. I loved it.
MW4: Mercs had also several of those aspects and also much better graphics, I knew what I was shooting and where I was hitting (I wasn't sure when I hit something with an AutoCannon in MW2), it gave me some freedom to choose planets, contracts, paths, infamy or nobility but I have the feeling that it wasn't deep enough. By then (thanks to MW2: Mercs) I knew what the whole FedCom civil war was about and who were the Clans, etc., but I think that without that knowledge the game would be as weird as MW2 refusal war was. The game felt kind of short. Also, only Specter and Castle were the only characters that were kind of developed and their motivations and past weren't very fleshed out. On the other hand, the missions had more "personality", on their own, it was more faithful to the official lore and of course, there was DUNCAN FISHER!
So, I'm between those two... and this one, of course.