Unfortunately the least pleasant of the stadiums you can visit on your trip is Oakland's Coliseum. It's really a stadium built for the Raiders NFL matches, which gets converted for baseball. Half the stadium gets closed off and covered in a big green tarpaulin when baseball is on and, because of the stadium layout, the field is actually a long way from the stands. Most players hate playing there. They have been talking about moving to a new stadium for some while - maybe now the Raiders are relocating to Las Vegas, the opportunity may present itself.
Easily the most picturesque is the San Francisco Giant's AT&T Park. Situated right on the bay, across from Oakland, it attracts the highest average attendance in MLB and is routinely full for matches. Occasionally a home run will fly right over the stands in right field and plop into the bay itself. There is even a scoreboard to tell the crowd how many 'splash hits' have been recorded! On the down side, I once watched a game there when an earthquake struck. The stadium is modern and built to withstand quakes so, rather than panic and run screaming, the crowd actually cheered as the stands quivered and the foul poles swung back and forth for a few minutes!!
San Diego's Petco Park is a modern, well equipped facility smack bang in the middle of town. Of the two Los Angeles stadia, Dodgers stadium is the most historic and, at 53,000+, is I believe currently the largest capacity in MLB. It has had a lot of renovation and improvement over the years but still feels like an older venue. Slightly younger, with a more modern feel and a bit more picturesque, is the Angel Stadium. I visited this last September for a couple of Blue Jay's games and it is quite a pleasant experience, However, the stadium is actually in Anaheim, which is about 40 odd miles from LA itself and is quite a trek through the notorious LA traffic by car. There is an AMTRAK station nearby, but I don't know what the service is like.
Given a choice I would go for a Giant's game but, as with most north American sports facilities, you are likely to have an enjoyable experience at any of them.
