Waveserpents.
You have 3 squads already that would benefit from a transport, and even if they had zero transport capacity, they would still be the best unit in the Eldar Codex, because of the amount of firepower they put out. Always give them a scatter laser, and always fire your shield.
Once you get up to 1,000pts+, there are very few Eldar army builds that won't need multiple waveserpents. -lots of our infantry are just too squishy and/or short ranged to be particularly useful without a transport.
General etiquette seems to be any more than 3 waveserpents is not very friendly.
Other than waveserpents, it kind of depends on what craftworld and/or playstyle you prefer, but the A-grade units IMO opinion are:
Warp Spiders
Jetbikes
Hornets
Wraith Knights
War Walkers
We don't really have any A-grade HQs, but best of an average bunch are farseer on jetbike, or spirit seer if you are short on points, going fully-mech, or are fielding wraithguard. There are some occasions where an autarch is preferable, but only if you going for massed reserves.
My $0.02