kk so some over points about Eldar in general and specifically Mech Eldar. This is probably ioverkill and I'll have 3 people respond before I do, but meh who care
Welcome to the wonderful world of Eldar. Everyone despises us, some think our tricks are over powered, some people downright hate playing us, and damn it a true Eldar player loves it... minus the Gay Eldar jokes that were overplayed 10 years ago.
We are a slowly dieing codex that loses another chunk of viability with every codex that comes along. As other have said there is a pretty large chasm in competitive play between our really good units and the other stuff. The one this that has constantly been untouched thus far has been our Wave Serpents, making them a rather focal point of the accepted best Eldar lists: Mechdar. Every passing codex another chunk of our armor becomes uncompetitive from the recent fall of out Wraithlord to the
DE codex (regular warriors treat you Wraithlord like a standard space marine) to the upcomming Grey Knight turning foot Warlocks and our beloved Eldrad into fodder via psycannons. All of our units can do good things in standard games, yes all of them, but in the competitive environment the lackluster units go from meh to liability.
The #1 rule of Eldar, every unit has a clear purpose. It can have more then one, as long as it does not impede on it's first responsibility.
- IE warwalkers stated above. Their goal is to overwhelming fire down a transport via side armor if you plan to outflank them.
EMLs give the benefit of raping clumped enemies, BUT they impede their primary goal by limiting your typical 24 shots to 6.
#2 rule, which goes for most armies, synergy. We have to rely on synergy hard to get us where we are going and typically this comes in the form of our Farseer powers. Guideseer + Falcon/Warwalkers/Prism/etc. Doom + Destructor/Harlie kisses/Nightspinner/Bladestorm. Fortune + Fullout/Seercouncil/Eldrad

. These are just a few example of how to use the powers with synergy, but our units have it as well. Fire Dragons + Banshees, Melta their face and tempt the idiot to get out to gun you down or assault you then next round eat his face.
Now a breakdown for mechdar in no particular order within the tiers because that is based on the synergy.
Tier 1:
Serpents
DA
Fire Dragons
Fire Prisms
Night Spinner
Guardian Jetbikes
Seer Council on Bikes
Eldrad
Farseer
War Walkers
Tier 2:
Autauch (sorry ol buddy. Too many points, eats an
HQ slot, just doesn't fill a role.)
Howling Banshees (no assault out of moving vehicles really cripples them and coupled with their point cost it hurts bad)
Defender or Flamer spam guardians (Cheap bodies are great, but their range cripples them)
Vypers (Someone please punch the person that made them ope topped)
Warp Spiders (Pricey and while fast other things can get the job done cheaper and better)
Wraithlord (We had some good times buddy, but newer codex just make ya a liability)
Falcon (Lots of points for things that can get the job done better :/ )
Tier 3:
Well... everything else since I'm only doing 3 tiers. I'll list a few that may need explanation for people though.
Scorpions: Good philosophy gone bad. Lots of attacks, but no power weapons and no fleet. This means they aren't viable to an assault-vehicle-less fast army since you need that fleet to make contact and in a world of
FNP and 2+/3+ armor dripping all over the place they just aren't going to make the cut.
Warlock Council on foot: You get out, you make a mess of a vehicle, you spend the next 3 turns running around like an idoit. 4+ invul for 25pts isn't cost effective either since even with fortune you die 25% of the time and with T3 that isn't a pretty combo.
Yriel: Yes, you are capt badass, but without a viable means of getting you into combat... sorry guy.
Harlies: 5+ invul with T3 and you can't take a dedicated transport... moving right along....
Shining spears: T3 3+ armor for 35pts... please see line above...
Reapers: You have no place in a fast world.
Hopefully this gives you a little more guidance, and to answer your question of who to bring farseer wise, if you have two falcons then hands down Eldrad since you get the two powers you need, plus a third, plus piles of perks. If one falcon or Bladestorm/Tri-Flamer then Doomseer w/ both runes.