Battletech is in upswing, lots of new players and the current era is ilclan. I'd bet in a lot of cases, the favorite era is the one you were introduced to.
Succession wars for me, bar none (voted too fast, not realizing you could pick two options, says it all). The heat scale, the characters, the setting the system and core concepts were intended for, and a fair chunk of my childhood "imaginary world".
Dont know what I would've picked as second choice between clan invasion, FCCW and the jihad...lore wise, I personnaly don't really diferrenciate, most of the cast is the same, there wasn't the same clear "passing of the torch" as between SW and CI...game wise, the older stuff is still revelant in Jihad era...but then I've never played table top beyond the 3067 toys.
Something I'd be interested in as far as era's are concerned is more "Age of War" (the gritty, no holds bared, part as far as table top is concerned). Perhaps the day TPTB revisits capital ship combat and planetary bombardments? Can't really imagine a greater delving into the AoW whitout table top big ship space combat making a push. Perhaps something manageable for Inner Sphere In Flames at the same time? Lord knows I've wasted time looking at the force tables in the old brush wars sourcebook and looking at the rules, but I can't imagine anything less then a group of dedicated BT addicts playing that. A box set to enable your planetary annihlation fantasys perhaps, so as to facilitate the process? Now I'm dreaming in color.