It's Taillon. I'll say this--if you're going to take a high school pitcher, Taillon is the guy to take. He throws very hard, he already has nasty hard breaking stuff, and he apparently also has a decent changeup. He's also a huge guy, and he doesn't have any outstanding health issues. There is no reason he can't be a top of the rotation starter in the majors. If/when he signs, he'll immediately become the Pirates' best pitching prospect and, after Pedro Alvarez graduates, their best prospect overall.
The question, though, is whether his upside is worth the second overall pick. The record on high school pitchers drafted early in the first round is notoriously awful, and there are so many things that can go wrong between now and his Pirates debut in 2013 or 2014 or so. Taillon is really a risky, reach-for-the-stars sort of selection, and normally I advocate high-risk, high-reward decisions for a team like the Pirates, but top-five draft picks are one area where I think they should be a little more conservative. To me that means drafting hitters. I would have taken Manny Machado (who went to the Orioles at #3). I'll root hard for Taillon, and of course he could end up working out great, but this pick just represents too much risk to really excite me.
You can watch video of Taillon here.
UPDATE: Some stuff from Taillon from around the web:
A good scouting report at Hardball Times. When I watched Taillon I had the same worry as the author here about Taillon leaving his fastball up, but this is obviously an issue that he'll have a lot of time to fix.