Ho-oh
I agree with Maniaclyrisist for the most part. Ho-oh is extremely strong, and with SR as it's only real negative factor it sends a troublesome statement to team builders: use SR if you want any hope of dealing with me. Due to the fact that SR is almost 'required' right now anyway (because of how difficult it is to cover many threats without it), I don't think this alone is a good enough argument to discourage testing. However, what I think will put it over the edge is that fire/flying is a great typing in the current metagame. For instance:
Scizor's Swords Danced Life Orb Bullet Punch vs min/min Ho-oh: 788 Atk vs 216 Def & 353 HP (60 Base Power):153 - 180 (43.34% - 50.99%)
Ho-oh only needs 32HP EVs or 20Def EVs to guarantee survival after SR. If carrying Leftovers, it doesn't need anything. Ho-oh will easily outspeed and OHKO with [fire move], so Bullet Punch is the 'best' option.
Scarf Heatran's Fire Blast vs min/min Ho-oh: 359 Atk vs 344 Def & 353 HP (120 Base Power): 67 - 80 (18.98% - 22.66%)
That doesn't come close to 2HKOing after SR. It needs a Flash Fire boost (and even then Ho-oh could guarantee survival without too much trouble if it wanted).
Infernape's Nasty Plotted Life Orb Fire Blast vs min/min Ho-oh: 614 Atk vs 344 Def & 353 HP (120 Base Power): 149 - 176 (42.21% - 49.86%)
Once again, survives after SR. Fire Blast is literally the best option both those pokemon have against Ho-oh without changing their sets. Maybe they'd start to run HP rock as a result, though. In Infernape's case, Lati@s' existence really hurts it's special sets anyway, so it would likely run SD sets more often than anything.
Those were just a couple calcs I ran out of curiousity. Even assuming SR, Ho-oh with no defensive EVs whatsoever can still serve as a useful check to some of the most used pokemon. No defense EVs whatsoever. Coupled with the rest of the argument (that is: how difficult Ho-oh is going to be to handle once it's in, and the whole 'forces SR' bit), I'm having a hard time believing that a test will be worthwhile.
The only interesting factor I could see here would be that suicide leads might become less popular due to the fact that spinning might become more reasonable as it would allow you to get Ho-oh in easy-style. That would mean people would start relying more on rockers who could actually come back in a few times.