Ten Letters

The most selfish
one letter
'I'

Most satisfactory
Two letters
'WE'

Most Poisonous
Three Letters
'EGO'
Kill it.

Most used
Four letters
LOVE
Value it

Most pleasing
Five letters
SMILE
Keep it

Fastest spreading
Six letters
RUMOUR
Ignore it

Hard working
Seven Letters
SUCCESS
Achieve it

Most enviable
Eight letters
JEALOUSY
Distance it

Most Essential
Nine letters
PRINCIPLE
have it

Most driven
Ten letters
FRIENDHIP
maintain it