To my perception, there are basically four gameable eras of what's now being called Classic BattleTech: the Star League (2750), the Succession Wars (3025), the Clan invasion (3050), and the Jihad (3067). Most of the material now being produced focuses on the latest of these eras. Why do I tend to favour the Succession Wars?
It's the first part of the game I played. In fact it was the only era available when I started to play BattleTech. I was there in Milwaukee in 1988 when the metaplot started and the Fourth Succession War kicked off. (I even got some of the wedding cake.)
I played it a lot. I designed a great many 'Mechs (most of which are now lost), got the hang of the system, and had a great deal of fun with it. I also had a fair chunk of background worked out for a mercenary unit, including the story of its founding (which isn't all that well-written but never mind). When the 20 year update came along, I felt let down: all of a sudden I was supposed to throw that away and start afresh.
It's morally ambiguous. None of the Great Houses was particularly a "good guy"; one could reasonably support any of the sides, at least enough to play forces belonging to them without feeling squeamish. When the Clans arrived, I had a lot of trouble finding anyone who wanted to play them, and I certainly wasn't interested myself. They had all the neatest technology, but that just made them munchkin bait. Even Mike Stackpole's Warrior trilogy, the first BattleTech novels to be tied into the metaplot, painted the ruling Davions and Steiners as heroic and incorruptible while the Liao were megalomaniac buffoons.
It's simple. None of this messing about with double heat sinks or extra-light engines, you have a simple tradeoff of speed vs armour vs weapons, and you can never fit everything you want into the chassis. Too simple? Perhaps, but I didn't stop playing because I was bored; I stopped when one of the long-term basic rules (that items couldn't be split over multiple locations - so the biggest guns couldn't be arm-mounted, and even the slightly smaller ones couldn't be combined with a hand on the same arm) was violated by FASA in one of the supplements.
So while I'll occasionally play with the Star League tech base (aka the Inner Sphere of 3050 and later), for most of my gaming it is forever 3025.