The most...

SELFISH 1-letter word 'I':
Avoid it!

SATISFYING 2-letter word 'WE':
Use it!

POISONOUS 3-letter word

Kill it!

USED 4-letter word

Value it!

PLEASING 5-letter word 'SMILE'

Keep it!

FAST SPREADING 6-letter word 'RUMUOR"
Ignore it!

HARDEST WORKING 7-letter word

Achieve it!

ENVIABLE 8-letter word

Distance it!

POWERFUL 9-letter word

Acquire it!

ESSENTIAL 10-letter word

Trust it!