I respect what you are trying to do but as a long standing Tyranid player I am going to disagree with all of these options for the following reasons.Tyranids: Anything with a lot of attacks and Rending Claws, Toxicrene for a funny and reasonably legit counter, massed Implant Attack if you feel lucky, Boneswords if you can roll 6s. Paroxysm taking it down to WS1 BS1 is amusing. Swarmlord with some Tyrant Guard to take the D for him should might do it if you can get Furious Charge or Preferred Enemy or something. Gargoyles with Blinding Venom.
1. Rending claws are good against it. If they can reach it. As you mention it moves 12 inches a turn so most creatures with them are not even going to catch it. Those that can (like Raveners, Shrikes and Genestealers) are either likely to be shot down on the way in (depending on the rest of the Eldar army and the load out of said Wraithknight) or get instant killed when they start slapping it in close combat or finished off via stomp.
2. Toxicrenes are NOT a valid counter to Wraithknights. Even assuming it reaches said Knight (since it moves 6 inches a turn) it only wounds it on 6's (since it is poisoned 2+ and not fleshbane). Now whilst it gets a decent number of attacks to do this and inflicts D3 wounds per 6 it has a much lower initiative than the Knight and will probably get slapped up by it before it gets to strike.
3. Implant attack no longer exists in the current Tyranid book.
4. Boneswords suffer the same problem as rending claws as explained above.
5. Paroxysm as a power no longer reduces the target to WS and BS 1. It instead drops both by D3 and since Knights were WS5 last book (I am assuming this does not change) then the best I can get it down to would be WS2 and BS1. Also I cannot assume that I will be able to out-psyk an Eldar army since they both are roughly the same on power dice etc.
6. Swarmlord will not even get close to a Knight. I'm sorry but no competent Eldar player is gonna allow that thing anywhere near his Knight. Not to mention the Swarmlord himself is nearly the same points cost as the Knight in the first place.
Looking through the Nid book I have come to the conclusion that there is almost nothing that can deal with a Wraithknight because it will either get shot down/outmaneuvered before it gets a chance to do its thing. The best thing I reckon stands a chance is to get a Hierophant up against it. Then again I find it ridiculous I have to field a 1000 point monster (which is still not SD I might add) to even stand a very good chance of toppling a Wraithknight.
Again this is not a personal dig at you (It's not meant to) but there is literally almost nothing a Tyranid player can do to stand up against it.