Counters...lol
First off I just want to make a bold statement here and say...no one counters Lucas.
Aside from that...the person I've had the most trouble with is Ike, just because Lucas is kind of light, and if Ike does happen to get off those smashes then boom...ouch. But other than one or two, fairly good Ikes, and bad timing on my part, good on theirs. I've had no consistant problem playing any of those characters you've mentioned.
Metaknight - ^smash kills his neutral B, usually kills him too...
Zelda - Lucas can shine with his Psi Magnet...yeah, no problems there.
Diddy - Shuffling fairs and nairs seem to take care of him rather quickly.
Snake - The Snake I usually play is fairly advanced, so I usually wait till he tries to drop me, place a sticky on my face...I roll dodge through him, to make it stick to him, then wait till he blows himself up, usually staying in the vicinity so I can air combo/^smash him. Just use PK fire to keep him far enough off, fair him if he tries to get back onto the ledge...
Marth - Use your spacing to gain advantage, wavebouncing the PK fire helps a lot. Or just throw out some running ^smashes, to get him in the air then use Bair to spike/repeat.
Peach - Wavebounce the PK fires, should keep her away, wavebouncing Psi magnet is good against her, because of the semi-shine attribute.
Toon Link - Watch for the Dair, hit him with a ^smash, shuffle nairs and uptilts to get him into position to try and dair, you'll have no real problem, unless they spam items. Just keep your distance and wait for an opening, Lucas can dodge fairly well.
All the others are either rather simple, or I haven't played them enough to develop a strat, though I haven't played anyone good out of the Wolfs, Olimars, Sonics, or Lucarios.
Keep distance from Luc and you'll be fine...
Lucas is one of the best characters in the game, don't bring him down with this counter talk...
~LOTF