1. 60 points - Target!
2. 45 points - The Hour That Never Was
3. 35 points - Death at Bargain Prices
4. 25 points - Fog
5. 20 points - Town of No Return
6. 15 points - Stay Tuned
7. 14 points - Too Many Christmas Trees
8. 13 points - The Morning After
9. 12 points - They Keep Killing Steed
10. 11 points - What the Butler Saw
11. 10 points - The Correct Way to Kill
12. 9 points - Sleeper
13. 8 points - Cat Amongst the Pigeons
14. 7 points - Faces
15. 6 points - Dressed to Kill
16. 5 points - Don't Look Behind You
17. 4 points - Girl on the Trapeze
18. 3 points - Mission to Montreal
19. 2 points - Bullseye
20. 1 point - Second Sight
But if you are not including the New Avengers, then:
1. 60 points - The Hour That Never Was
2. 45 points - Death at Bargain Prices
3. 35 points - Fog
4. 25 points - Town of No Return
5. 20 points - Stay Tuned
6. 15 points - Too Many Christmas Trees
7. 14 points - The Morning After
8. 13 points - They Keep Killing Steed
9. 12 points - What the Butler Saw
10. 11 points - The Correct Way to Kill
11. 10 points - Dressed to Kill
12. 9 points - Don't Look Behind You
13. 8 points - Girl on the Trapeze
14. 7 points - Mission to Montreal
15. 6 points - Bullseye
16. 5 points - Second Sight
17. 4 points - Death of a Batman
18. 3 points - Castle De'Ath
19. 2 points - The Wringer
20. 1 point - Brief For Murder
Sole criterion is how much I enjoy watching (and rewatching) them.
Here's a (very rough) take. It's just so hard to rate them when there are so many wonderful episodes that I enjoy for so many reasons, but here we go:
1. Death at Bargain Prices
2. Faces
3. Target
4. Honey for the Prince
5. What the Butler Saw
6. Dirtier by the Dozen
7. The Midas Touch
8. House of Cards
9. Who's Who?
10. The Quick Quick Slow Death
11. Game
12. Who Was That Man I Saw You With?
13. Take-Over
14. The Murder Market
15. The Charmers
16. The Hidden Tiger
17. Forward Base
18. The Wringer
19. The Nutshell
20. Intercrime
The Positiive Negative Man
Too Many Christmas Trees
The Living Dead
The House That Jack Built
Return of the Cybernauts
Fog
From Venus With Love
Warlock
Don't Look Behind You
Mission...Highly Improbable
Silent Dust
Mandrake
Pandora
Requiem
Town of No Return
Stay Tuned
Epic
Brief For Murder
The Hour That Never Was
You Have Just Been Murdered
This was a very hard choice and as others have said might be totally different tomorrow, but its the order I'd watch them if I was having a marathon viewing today/