Dire Avengers - Great unit, love them with blade storm, I find them swappable with Storm Guardians depending on the field (amount of cover) and enemy (GEQ or MEQ)
Banshees - Never had resounding success with them, the lack of assault vehicle has always been my main problem, anyone that knows Eldar can easily avoid or set up the Banshee Serpent. Having said all that, never had resounding failure with them either just middle of the road.
Prisms - Almost too much scatter these days, and easily too much cover, makes them unreliable imho, they can't tank hunt easily because of scatter, and the small blast is too easy to miss 2+ save units (and with SS/TH or cover, its almost blah). I love the models, and they are still a decent tank, just not as decent. I usually prefer putting more serpents on the field.
Bright Lances - Never work for me, too expensive, only marginal effectiveness (worse then a las on AV12, equal on AV13, and 1 better on AV14) BS 3 ruins them, take EML's instead, same strength, only a little less effectiveness, and better point cost. However if you do this, you need Dragons to take on high armor targets.
My opinion, drop the prisms (even potentially the falcon) and put in 5-6 man squads of dragons in serpents, better all around unit. MC's, FNP and Tanks fear the Dragons. The number of serpents on the field will make it hard for your opponent to pick targets, just try to get a squad of avengers deployed someplace nice early in the game to give them even more targets to worry about.
Best Weapons: EML's and Scatter Lasers are easily the best bang for your buck in the dex, Shuri Cannons can be great, but the 24" range means more finesse, so I don't usually recommend them until you have some experience with the army.
Best Unit: The winner is almost always Dragons, an entire squad of melta weapons and cheap. They do usually die the turn after deployment unless you deploy late or in a covered position, but used well they will always return point cost before death.