Never ever EVER purchase leech. Ignoring how "weak" it makes you, it wastes money.
Unless you have access to a bishop, don't leech. When I sell leech, I feel like I am stealing from the people I sell to. But who cares, they're desperate enough to pay that much.
I also suggest you play a character up to 120 before leeching anything else, you lose out on so much if all you have done is leech.
As for your specific question, Harps leech is the best from 78-90 or so. After that, Wyvren is okay if you 1-hit, and then Newtie at 105 and Skele at 108.