I wouldn't take Mewtwo over current Dante and I don't think Lugia has a prayer at beating Mewtwo, so I don't think Dante vs. Lugia would be close.
Lugia should probably be the strongest Pokemon GSC has to offer, though, and while we've seen stuff originating from RSE and DPP there's been nothing from GSC. Lugia probably would be a midcarder and it should be well ahead of Lucario and Bidoof.
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[NO BARKLEY NO PEACE]