Personally I love the inclusive TRO style that we have now. Getting snapshots of everything new, all in one book, it gives the world a nice richness. You also get to see how entire armies evolve in their hardware as threats show up - look at the proliferation of various technologies and their defenses, all in the same book. TROs got my instant vote. The rest...geez. It's hard to say at this point...Field Manuals are, for the most part, full and done. Between the FMs and the HBs we've got tremendous amounts of info on the major factions (that anyone cares about, there's those samurai guys but who likes them ;D) already and I don't see much need short of massive changes to anything.
Like say, what happens to the Lyrans if the end of Bonfire becomes a permanent situation with Alaric. Or the reformed FWL, if the Zombie State is significantly different than what it was before, or the major post-Reaving situation "up north" - after 75 years, the post-3150 Scorpions and the Cluster ought to be radically different than it was. Or if we go back in time and do something for the Star League itself. But outside of that, I can't see much of a need for Field Manuals; the groundwork is laid pretty thoroughly.
The way the BT universe goes, I'm definitely digging the Era Reports and the Historicals, but the Plot Sourcebooks like the HS series and the War of Reaving are absolutely amazing. Frankly, between the HS series and Ben's .44 Magnum Opus I can't find anything I don't like, except the lack of MOAR. I went with those more, because of the general quality and density of product.
And Technical Readouts...hell if you keep the current art crew and the designers, I'll give kidneys for it. I might even give one of my own. BT has, at its heart to me, been about the 'Mechs - and I am, admittedly, a very visual person. 3025 hooked me into the game through seeing giant stompy robots, and the Catalyst TROs have been immeasurably better. BattleTech is a game, not just fiction - and we need new dudes to play with.
I love it all, but I picked the Plot Books and TROs. That was, admittedly, a bloody tough call.