[{quoted}](name=Inept Maverick,realm=NA,application-id=3ErqAdtq,discussion-id=Ayee51ZF,comment-id=000000000000000000000000,timestamp=2016-03-10T23:49:09.176+0000)
My usual Middle Lanes I play are:

She counters all those champions (if she is good)
Karthus is the easiest possible matchup, even for a no skill LeBlanc-Player. She has enough ms to dodge his q's and Karthus does not have the range to outpoke her. Only with a perfect wall and the other skills spammed he could be able to do any damage to her (if she has no w, which is really rare when she maxed it out)
Veigar can be strong, but an experienced LeBlanc will be able to play around his cage. Only his ult combined with his cage and a w is effective, and especially in midgame veigar is able to oneshot LeBlanc easily.
Vel'koz is also really effective if played well. For the opponent it is a dodge-minigame. He is good because he can push the lane very hard and LeBlanc is bad at farmng under the tower in early-midgame if she do not wants to waste her mana/spells. When LB engages with one w (and uses her qre to do damage), spam every spell, the ult is able to slow and to hit her even on the return bad (same with the other spells)
Xerath is same as vel'koz, but seems easier to dodge. The Xerath player has to use his pushing-advantage and his stun with good reactions to win the lane. Only an experienced LeBlanc could handle this matchup (if the Xerath is good).
Ziggs is another easy matchup for LeBlanc. His only advantage in the laning phase is his q poke. His waveclear is not extremely good in early game and LeBlanc can easily harrass with her q+w. After two items the ziggs can be more useful than a LeBlanc because his ult is soooo strong in a teamfight and the other abilities do aoe-damage.
The most information I wrote is meant for the laning phase. Later every mages will be more effective and they offer more utility than LeBlanc. All of them do have more damage and teamfight potential, not to mention their insane waveclear. Vel'koz is also effective vs tanks, LeBlanc is definitely not.