The list starts to get a bit odd here. First base has become the place to put players who can't really move. Very few players are actually first base prospects, they just end up there when they can't handle third, catcher, left, etc. Take Albert Pujols. He came up as a third baseman, but was eventually moved over to first. At first, he's actually one of the better athletes playing the position. At third, he was a liability. Ryan Braun would be a first baseman right now if not for Prince Fielder. Matt LaPorta would be one of the top first base prospects in the majors, if he wasn't a Brewers prospect and going to be forced to play right (most likely) or get traded.
The top 1b prospect under 23 fits this same profile. I mentioned him in my catchers entry, because he's a converted catcher who couldn't quite cut it at the position.
Unfortunately, the top 3 young first basemen are all ineligible for this list: 1) Prince Fielder, Brewers, 2) James Loney, Dodgers, and 3) Joey Votto, Reds, in that order. Power is one of the later tools to develop, and it's not optional for a first baseman like it can be at other positions.
1) Daric Barton, Athletics, 8/16/1985, 22
Barton was drafted in the first round by the Cardinals, as a catcher. He couldn't stick at that position, unfortunately, as he'd be by far the #1 hitting prospect at that position. Regardless, he looks very promising and completely fits the A's profile of high OBP players. He had a good, not great, season in AAA, but was amazing in his September call up. Something in between should be expected for 2008, assuming the A's give him the first base job (here's a hint: they should).
Barton's an example of Beane's good trading. The Cardinals look somewhat foolish now for their deal--Dan Haren, Kiko Calero (a useful but not stellar middle reliever) and Barton as the PTBNL for Mark Mulder. Mulder gave the Cardinals one good season before breaking down--a season that was not quite as good as Haren's season in the same year. Haren gave the A's two very good and one great season before being himself turned into Carlos Gonzalez (who in turn made Nick Swisher expendible), and six other players whose names we may not know today, but undoubtedly will hear about soon.
ML Stats: 72 AB .347/.429/.639, 4 HR, 8 RBI
2) Chris Marrero, Nationals, 7/2/1988, 19
When a player is listed as a 1b/of as a prospect, you might as well drop off the "/of" from his list of eligible positions. If there's question as to his skill in covering the outfield, he won't last there long. That said, his power is not in question. As an 18 year old, he managed to hit 23 hr in A-ball (14 in low-A, 9 in hi-A). He's not walking enough, which is of some concern, but his strikeouts aren't completely out of line. If he develops a little plate discipline, he'll be excellent.
A ball stats: 477 AB .275/.338/.484 23 HR, 88 RBI
3) Lars Anderson, Red Sox, 9/25/1987, 20
What? But he plays the outfield! Yes, he sure does. He's already a below average runner, though, and he's going to continue to thicken as he approaches 25, and will eventually need to play 1b (in my opinion). His bat will play there--his power is tremendous, and his swing, even now, is not bad for Fenway. The worst thing about him as a prospect is the team he plays for!
A ball stats: 493 AB .292/.393/.446, 11 HR, 78 RBI