There will never be a definite answer to this. It depends on your own personal choice, the appearance of your favourite character, how do you like the episode to be (funny? scarry? confusing? emotional?). Some people will say that they all good. But some are definetely better than others. In my opinion, here are the list of the episodes, from the best to the alright ones:
01."Sisters"
02."Pilot"
03."Reunion"
04."Love and Death"
05."5D"
06."Send Me an Angel"
07."The Many Faces of Alex"
08."Escape"
09."Dr.Strangechild"
10."Control Freak"
11."Parralel Lives"
12."Simon's Choice"
13."Facing The Fire"
If you ask me why I choose "Sisters" as the number one, I would say because it's the most original and different episode. It is the only episode with no conspiracy stuff whatsoever and it is a nice break to the usually tense episodes of VR.5.