I am a Battletech fan, who was first introduced to the universe by the novels (Star Lord was my first) and then the PC games (Mechwarior IV). Ironically I have never played the original hex game, but I did try the computer version. Sadly I was beaten on all three occasions due to unfamiliarity with the rules.
Well, and this may be a shallow answer, I like giant robots blowing stuff up. Technically a 'Mech isn't a robot (cos it needs a driver), but you get the idea. I think a 'mech would be considered a "humanoid vehicle". The novels were pretty okay too. Sure, it's not fancy-pants literature, but it was engaging enough for me. Some politics, some action, a little romance, plus giant humanoid war machines is all you need.
Of all the units, I actually liked Camacho's Caballeros the most, and I don't even know if they're canon (they were in the Close Quarters novel). They seem more like wandering cowboys than a merc unit, and that's just cool.
As for 'Mechs, the BNC-5S Banshee is my choice. Not because it's particularly powerful, but because I had a copy of its card from the BTech CCG. I used it as a bookmark for my BTech novels. Then I lost the card, and had to use the BNC-3E instead.
 CBT/Dark Age
I have mixed feelings about the Dark Age storyline. At first I thought the lack of 'Mechs was a bad things, but it actually enforces the idea that BattleMechs are rare and special machines. The combined-arms approach, using a 'Mech as the center of the formation, is given more focus in Dark Age, and I actually like that (maybe I should join Hells Horses). I don't like how they created an important character like Devlin Stone and subsequently disappeared him without explaining WTF he was. I'm surprised that Victor Davion had to be murdered. I had expected him to be already dead by then.
The game itself seems okay. I'm more used to using rulers instead of hexes due to my experience with WH40K. Also as I mentioned I never played the original board game. I have however attempted to understand the rules by downloading the various 'mech design programs and hanging around this wiki. I think I got most of it so far.
The Ares is another thing altogether. What role does it have in the universe? It may be big, but an assault lance should be enough to be rid of it. Just read the weapons loadout on this wiki. It's not that impressive, really. Unless they're going to handwave away its strength by saying "experimental technology". I wonder if it can also fly or teleport, because for such a humungous thing to maintain a veil of mystery that it does is kinda hard to pull off. And why three legs? Four should provide an even more stable platform for extra weight. It's walk cycle is probably all kinds of wrong. No creature in heaven or earth has three legs.
 Random Questions
WTF happened to Arthur Steiner-Davion? Are X-Pulse lasers canon? Was the WOB Jihad pre-WizKids or post? Why haven't human beings encountered any black holes after exploring so much space?