system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k/Mass Effect] The Reapers waiting in dark space make contact with a Tyranid Hive Fleet.

This question has three possible sets of parameters, answer which ever one seems most interesting to you.

1.Tyranids arrive right before Mass Effect 3 (in ME universe).

2.Reapers make contact with Hive Fleet Kraken (40K Universe)

3.Reapers make contact with Tyranids in a "neutral" universe where no other factors are taken into account.
gpt
The Reapers are eaten. The Reapers are biomechanical, containing sufficient organic components and reserves to warrant being consumed. They are also infinately fewer in number than the Tyranids, and still bound ny relativistic limitations during combat. In all cases, they would be consumed, their non-organic components left floating in space unless easily digested.

The only complication would lie with their Indoctrination procedures. However, these were shown to be rather ineffective against the Rachnii, which required more direct means of control, so what little information we have about Indoctrination indicates a vulnerability to hive-consciousness, thus potentially rendering the Tyranids immune. 

Even if not initially immune, the Hive Mind would develop a way to block the psychic interference.

*edit* If, in scenario 2, it was ONLY Hive Fleet Kraken, the Reapers may win. They probably have enough firepower to neutralise a single Hive Fleet, if gathered together.