Currently, there are three legendaries with Solar Beam (SB) - Ho-Oh, Groudon, and Latios. Here's my analysis on them:
There are three types that are weak to grass (SB). Rock, ground, and water. Of the three legendaries, Groudon resists Rock, Ho-Oh resists Ground (from flying's double resistance despite fire's weakness), and Latios resists Water.
So my thinking on this is that SB Groudon should go against Rock, SB Ho-Oh goes against Ground (like Groudon), and SB Latios goes against water (like Kyogre).
Now, I doubt that these are the best counters for these opponents, but if you're going to use Legendaries with Solar Beam, that's the way to use them. Solar Beam is a really strong, albeit slow, attack that doesn't need STAB to be effective.
Thoughts?