Sort of not useless, but its becoming to be -
MCH's Knockback ability.
Reason:
The skill level of PVPers wised up to it "sigh".
Ok jokes aside actual skill I found useless:
BioBlaster
It ruins my groove every time it comes up, and I wish it would be a separate skill instead of being in rotation. I rather not try to get up close to players because there is hard potential to get stunned into infinity and get ganked.