Never went to River Country! I was still vacationing, not living, in Orlando and I did not make time for water parks! I was like, "We can go to water parks at home!" The other two...
That is a tough choice to make! If I just want to lie around on the beach, do the lazy river, and have a relaxing time in the wave pool, either will do just fine.
If I want the activity of swimming and/or the giant waves, Typhoon.
Blizzard, in addition to the many many wonderful slides and play areas for kids, has the better thrill slides for older kids/adults. They have the better family ride, too, IMO.
Typhoon has swim with the sharks and the giant wave coming through the wave pool. The giant wave, when it gets to the entry part of the zero-entry, is still cool for babies and toddlers.
But they tiny tots can have fun splashing the zero-entry part of Blizzard, too.
Wee ones seem to just enjoy splashing in the water.
(And they're so cute!)
Toddlers, babies, and younger pre-schoolers, I'd probably pick Typhoon. Kids, in general, I take to Blizzard first. There is just more for them to do there...more easy slides for younger kids who don't do the thrill slides, nicer play areas for young kids. Older kids/thrill slide slide people, totally Blizzard.
Personally, for me, it usually comes down to where I am when I head to WDW. I go to whichever is closest! If I'm leaving from home, that will be Typhoon, but I'll sometimes drive down to Blizzard, even so.