I kind of want to unleash the Mass Effect Reapers on several other Sci-Fi galaxies and see what happens.

The break-even point is probably somewhere around Star Wars level, I think. Star Wars could beat the Reapers, but it would be a fight, not a walkover. It'd probably come down to how many capital ships Star Wars has available. Reaper Dreadnaughts are tough, but not invincible. A Star Destroyer could likely take one on.

Unleashing them on Star Trek could be interesting, it wold be a much nastier fight, but Star Trek does have its own evil synthetics issue in the Borg. Oh, now there's a fun idea. Replace Borg with Reapers, see what happens next. Plus, "You exist because we allow it, and you will end because we demand it." is such a great boast, better than the Borg line of "resistance is futile". A Reapers in Trek crossover would probably be more of a battle of wills between Picard and Sovereign, and I'd be okay with that.

Warhammer 40k? Eh, just one more race bent on galactic domination. They'd have to get in line. The idea of Ork Husks alone would make it fun, though. Heh, on the other hand, the Reapers could be written in as the thing the Tyranids are running from. That'd set them up as a major threat.