I never had the opportunity to play Mechwarrior when it was at its height, but I do have my favorites from when I first started and now.

Shiro's- Hardened Armor and an awesome range of 18. Although a little slow, still very dangerous, even without their specific pilots. As a matter of fact, I prefer other pilots because they start out with low attack values. I find their sweet spot to be somewhere after they've taken about 4 clicks.

Veteran Nova Cat- The Clan Nova Cat Nova Cat. Another big range (16) heavy mech who can both take and throw a punch. The ability to "freeze" an opponent with a hard hitting tsemp shot is pretty awesome.

Jackalope- I don't really care that they can't really throw a punch and can't really take a punch. A nice DFA from so far away by one of these guys on some support units can really make my day. That, and I always got a kick out of the way they look.

Thunderfox- These guys look cool and are actually very effective, especially when either hindering or water terrain is involved.

Solitaire- 'Nuff said.

Havoc- I don't really care for the terrible heat management, but these guys sure do pack a whallup.

Then:
Jupiter- Specifically OEJack and Erin Johnston. Couldn't really top those two. Sorta of plodding slow, but still laid waste to opposing armies.

Bounty Hunter- 12 attack and Evade on an assault? I'll take it.

Points for looks: The Clan Jade Falcon mechs from the Falcon's Prey set. Shrikes and Gyre Falcons look way cool, and aren't slouches in combat either thanks to the most awesome heat dials and vent ratings.

Arbalest- Little machines with a big bite. Long range and decent heat dial. Don't really need to say much else here.

Mad Cat III- Although there are only two, Kendrick and Mason, they are both excellent pieces. Never lost either of them in a game.

How about the rest of those who occasionally lurk here on the forums? Which are your favorites?

I've always liked the Ryoken II, at least for looks. I have one I modified for home play, replaced forearms with SM1 Tank Destroyer guns.

For playing with

Dragon's Fury Forestry Mech Mod for early on 300 point games. Best 100 point mech in Dark Age to me

Banson's Raiders Bart Bradshaw seemed to be everyone's favorite at the time. I think I ran the Locust mostly, paired with a Legionaire and backed by Kenneth Medchelk.

Always use the Dragon's Fury Jupiter when I can, even though I haven't had the best luck with it.

I mostly play Storm Hammer and House Steiner when I can get a game. Explosive mechs makes for tough choices, almost make Swordsworn heat dials look playable.
Either Eisenfaust (spelling?). Both are solid for an army that's strength is artillery.
SH Hell star. 5 pulse and won't shut down on first 2 clicks, gotta love it.
HS Mad Cat mk IV. Hits hard and OK armor for points
HS Jackalope. A unit that moves faster than 8 and won't shut down on first click

I'd have to go back through my armies to see what else and why. There are a number of mechs I like the look of that I won't play because of stats, SH Sphinx for one(no range, no armor, no heat dial) and SH Warhammer IIC. I do like Nova Cats, Crimson Hawks, the first Atlas, Jupiters, Thors, Mad Cat & Vulture mk IV. Too many units to remember all of them and my thoughts on each.

I still have only a fledgling Stormhammers force, but it is a heck of a fledgling force led by Michaela Freeman and a veteran Rifleman. Talk about firepower. I don't think I would go near either the Warhammer or the sphinx. I do have a decent Steiner force. "Scion" Eisenfoust is definitely one of my favorite mechs, especially when backed up by a jackalope or two. A friend of mine calls the jackalopes "those dang ninja mechs".

I've never acquired either the Stormhammer Hellstar or the Steiner Mad Cat MK IV.

I'm more of a Steel Wolves and Rasalhague Dominion Player, who occasionally dabbles in Republic units, but I have at least modest collections in almost all factions. The only Mad Cat mk IV I currently have is Anastacia's Alpha. With a Vengeance is one heck of an ability, but one must be very careful about the timing when using this ability.