Trek is known for its "forehead of the week" aliens, or their "tweak-a-small-feature" aliens (not even counting those alien races who happen to look exactly human, even in TNG time frame.)
My vote for the "best" would be the Yridians from TNG. They do seem to be the most "alien" looking aliens on all of Trek, considering their limitation of only human actors.
For the worst, I used to name the Andorians (typical 50s/60s pulp alien archetype), but they've grown on me since they were updated for "Enterprise." Now, I don't think you could get any worse than that alien backwards-talking smuggler who tried to help McCoy in TSFS.
Others?
My vote for the "best" would be the Yridians from TNG. They do seem to be the most "alien" looking aliens on all of Trek, considering their limitation of only human actors.
For the worst, I used to name the Andorians (typical 50s/60s pulp alien archetype), but they've grown on me since they were updated for "Enterprise." Now, I don't think you could get any worse than that alien backwards-talking smuggler who tried to help McCoy in TSFS.
Others?