Some of the following have been mentioned already, but I make no apologies because they're so good. :-)
1. Space Fall: Blake working so hard to persuade Avon to help him while Avon has already decided it's his best option but makes Blake sweat for it anyway.
2. Time Squad: The first flight deck fireworks.
3. Seek-Locate-Destroy: Rontane and Bercol. Also that first scene with Servalan and Travis.
4. Bounty: Vila preparing himself to carry out the "personal investigation".
5. Deliverance: "That seems a poor reward somehow." Avon showing unexpected humbleness.
6. Shadow: "The next one will go right down your throat!" Cally at her most ruthless.
7. Horizon: Avon's dithering until he has a face-saving reason for rescuing the others.
8. Pressure Point: Gan's death. Not so much the fact that it was Gan, but that one of the heroes could die and stay dead.
9. Trial: "Avon's gadget works!" And Avon's reaction.
10. Gambit: Avon and Vila taking the opportunity for some naughtiness behind Blake's back.
11. Star One: "Be informative, and I may let you die. You'll want that after I've shot off an arm and a leg or two." Avon at his most ruthless.
12. Star One (again): "Avon, this is stupid!" "When did that ever stop us?" Sums up the series.
13. Volcano: Avon deceiving Mori with his "guided tour of the Liberator".
14. Volcano (again): Avon fighting a space battle on his own and being comforted by Vila after getting wounded.
15. City: Bayban being miffed about Blake beating him as most wanted criminal.
16. Sarcophagus: Avon being considerate towards Cally - in his own unique way :-)
17. Sarcophagus (again): Avon defying the alien. "Make me die. There's nothing else you can make me do." On first watching this I was quite shocked that Avon was so brave. :-) (Oh, those fond memories of watching the show for the first time and not knowing what will happen next!)
18. Assassin: Avon telling Tarrant he'll back him, then sitting back while Tarrant gets a beating.
19. Warlord: Soolin's smile when Avon threatens to sacrifice her if his alliance with Zukan requires it. She's not impressed.
20. Blake: The last five minutes make the whole series unforgettable.
And much, much more.
Marian