I'm surprised no one has mentioned the Scarabus yet.
The Scarabus, while okay for its day, doesn't age well. If it doesn't get into melee the amount of damage it can deal is inconsequential. It's still useful outside of a duel, but unless you can get a good hit with hatchet it is underwhelming.
In my Alpha Strike forces I have quite a few Koshi 3s (the new versions) I love those fast little buggers!! Yes they are squishy but so much fun to run around like a kid after 4 pounds of Halloween candy!
After that I like the Solitaire. But it seems too good to me usually. Not "struggling merc" enough. ;)
The Koshi 3 is a bit to squish for my tastes in a duel. It's a good lance/star mate when it can be ignored in favor a larger target. The fact that it can be disarmed with a couple shots of a Large Pulse/SnPPC makes it unattractive to me a dueling mech. I enjoy a Solitaire, but as Ogre mentioned when facing another fast target that +1 to with with heavy lasers is a big factor in who wins of losses a match. Both the Solitaire and the Scarabus could use a good DA upgrade.
What about a mech with a bunch of Re-Engineered Lasers? Ignore whatever screwy armor your opponent wants to use?
If you get into custom mechs I suppose you can make one, but the value of that mech comes down to entirely what your opponent is running. Looking at the existing designs there are a handful of mechs that take advantage of special armor types that ReLas would fielding against. The designs that worth playing are the Gunsmith, Hollander III, Urbanmech, and Silver Fox. Counter to that ReLas designs out there are the Ebony, Spider, Nyx, Prey Seeker.
None of those ReLas designs are all that impressive and have much weaker armor profiles. The Ebony and the Spider make the most sense of the two to use as they feature a Large ReLas. That is more likely to make a difference with the longer range than the other 2 which have no range advantage at all. I'd put my money on any of those special armor mechs 100% of the time to come out ahead against those ReLas armed mechs.
While I'm at it, I should say that the Ebony MEB-9 is a nice little dueling machine as well, speed, stealth, snub, and TSM if it gets warm enough to really do some damage to something in melee should it have to deal with a target that has reflective armor.