I am personally a big fan of Brandon Belt. I get that he has his problems, but he plays the game the right way, and he has a ton of potential that he is slowly growing into. He has the tools to become a great defensive first baseman, with a career .993 fielding percentage, and he can also draw walks, and has the potential to hit for power and average. He could be great, but there are definite question marks in his game. He had some issues with striking out, and was very streaky during the season. He showed glimpses of raw power, hitting a home run in three straight games in June, but he also had a homerun drought to start the season that went all the way until June. He's very interesting, and I wouldn't be surprised to see him put it together next season. He was not bad this season by any means, batting .275/.360/.421. I'd like to see more slugging out of a first baseman, but over time that should get better, and his OBP is definitely something to look at with optimism.
Beyond that, the giants farm system is a little low on first base prospects. The two biggest being Ricky Oropesa and Angel Villalona. Both of these guys are like Belt: there is serious potential but there are also big question marks. personally I would say Villalona is the top prospect, with Oropesa being second, but you could put up an argument for the other way. I mostly say that because Villalona has a little more potential and is younger.
Angel Villalona was on his way to possibly an incredible career with the Giants as an 18 year old in San Jose, until his career was put on hold following a murder trial in his home country, in which he was a suspect. After a few years stuck there, the charges were finally dropped, but he then had trouble receiving a visa to come to the United States. Luckily, he was able to come and play part of 2012 in the Giants farm system in the DSL. In the DSL, he hit .303/.430/.497 with 7 home runs and 34 RBI's in only 44 games. That's definitely not bad for a guy who's just getting back into the game. Even though he spent a few years in jail, he is still young for a prospect, only 20 years old, and he was protected from the rule 5 draft by the Giants when he was added to the 40 man roster earlier in the year.
Before he was arrested, a scout wrote this about him:
A big, strong first baseman with plus, plus raw power from the right side. Has a chance to be an above-average hitter but will have to watch conditioning, which was much improved over the course of his first full season. Made a nice transition from third to first base.
He also said this about his potential:
An All-Star first baseman who hits in the middle of the lineup and hits 30-plus homers and drives in more than 100 runs annually.
So he was definitely highly thought of before his arrest, and at such a young age, he will still be able to tap into that potential. His bigger question marks are his conditioning and questionable defense, but those can both be improved. I would expect to see him in San Jose this year, but I would't be surprised if the Giants move him quickly from there.
Ricky Oropesa is another interesting prospect. Described by some scouts to have "light-tower power", he could potentially be a huge threat in the middle of the Giant's lineup. He was drafted in the 3rd round by the Giants in 2011 and got his first full season of organized baseball at San Jose last year. His first full season was a little disappointing, considering he only hit .263/.338/.425 with 16 home runs and 98 RBI's in 134 games. It wasnt too bad but I'm sure more people would have wanted to see quite a bit more homeruns, but his RBI's were pretty good. He could also work on his strikeouts, he had 150 in 518 at bats last season. Power prospects are also known for developing late, so Oropesa is far from done. I would expect to see him at AA next season, and I think the fall league was good for him to see some more advanced pitching.
Overall, any one of these three guys could become good, possibly great, major league starters in the next few years. Belt is pretty much there, but could definitely use some tweaks. For now Belt will get a great opportunity to start on a championship team, and Villallona and Oropesa will continue to develop.