There's no "once upon a time" for this position. This is a position that was constantly filled with veterans and trade acquisitions for years. Will Venable stands out as the outfielder the Padres have produced since pretty much Tony Gwynn and Kevin McReynolds. The others on the major league roster are still veterans and trade acquisitions. However, their time is coming to an end in San Diego and it'll likely be the depth of the system that will have to step up.
Promising: Cameron Maybin
Grade: C+. An OF with Maybin, Venable and Denorfia plays solid defense and each have had quality seasons at the plate. But putting it all together into a productive OF doesn't seem to work on a daily basis. Quentin doesn't provide the defense, but can provide a bat. When healthy. When, if ever, he is healthy. Smith is a quality role player. Denorfia, and Smith will both be free agents come the offseason, which significantly decreases the depth of an outfield that relies on platoons for success. Venable and Quentin will be free agents the year after leaving some big holes in the corner OF spots. Only Maybin has a chance to provide longer term stability, but he needs to be healthy for that to be an asset.
Depth: Rico Noel
Grade: D+. This looked a lot more promising before Dickerson' injury and Fuentes' demotion. The grade is assuming Dickerson can return to the OF once his foot problem is handled surgically (although given the depth at 1B, a move back there would be an organizational upgrade). Now, they can't even fill an OF with prospects and the one promising prospect is probably a 4th OF at best.
Top: Rymer Liriano
Grade: B+. There we go. A full OF of prospects plus an extra for depth. Liriano looks to be on track to supplement the big league roster as some of the current squad go to free agency. Jankowski was trying to show that his speed and defense resume could play in the upper minors, but then he got hurt. That won't answer any questions about his bat. Fuentes excelled in AA last year and got a big league promotion, but now has to repeat the level after some struggles in AAA. Gaedele has a better bat, but is older and doesn't have the speed or defense. Asencio just borders on depth.
Top: Hunter Renfroe
Promising: Cory Adamson
Depth: Alberth Martinez
Grade: B. The prospects of the Lake Elsinore OF are very similar to the Missions OF. The difference being that Martinez looks to be a better prospect than Gaedele or Asencio. Aussie Adamson is actually the youngest of any OF mentioned so far and has been growing up slowly since being signed as a 16 year old. Hunter Renfroe may be an elite prospect a year from now. He is the premier slugger in the entire system.
Promising: Franmil Reyes
Depth: Mallex Smith
Grade: C+. Not all "promising" prospects are equal on this list. Most project at best to be 4th or 5th OF types in the majors if things go well. If things go well for Reyes, the sky is kinda the limit. The 18 year old is starting to see his bat develop like scouts had hoped. He's a big boy at officialy 6'5 240 lbs and as I mentioned in the corner infielders post he might end up a 1B. Mallex Smith also could be better than just depth. He's a base stealer like Jankowski and could be where he is in a couple years.
Depth: Rod Boykin, Jhonatan Pena
Grade: B-. Baltz is out injured, but appeared to be a promising bat before he collided with another player during Spring training. I don't know where he would have ended up (either with the crowded OF in Lake Elsinore or the crowded OF in San Antonio), so he goes here for now. Urena got a bit of hype before the season and entered the Midwest League as one of the younger players. After a couple weeks with Fort Wayne the org sent him back to the instructional leagues to work out some kinks. Boykin is a base stealing CF like many the Padres now have. He just turned 19 last month. Pena just turned 20, but had a nice season in the Dominican league. I may be off base about him being a prospect, but since he can play CF I'll overlook his age and see if he shows us something this season.