I have not visited many MLB stadiums so far but some of the ones I have visited were well worth it. One of the most distinctive I have visited was the Boston Red Sox Fenway Park. I got to visit this with my wife years ago and then again last year during its 100th year celebration. This time instead of taking in a game we did the Stadium tour. I highly suggest this if you are ever in the area. Walking on the Green Monster and sitting up in the booth is a pretty awesome experience. The tour guides are very good. Learned things I never would have known. Such as there is not a bathroom inside the Green Monster and the folks that maintain the old fashioned scoreboard must sit through entire games out there working the board without leaving.
I also got the chance to catch a game in the old Astrodome before it ceased being a MLB park. It was weird and interesting at the same time watching a game in the world's first domed stadium. It felt unreal watching an MLB game in the air conditioning while it was 100 degrees outside.The first home run ever hit there was by Micky Mantle he also had the very first hit.
The last of my favorites is Rogers Centre (aka the Skydome) where the Toronto Bluejays play. Watching the roof retract right before the game was very cool. Then watching the game and seeing folks in their rooms at the Marriott Renaissance Hotel also watching the game was amazing. I believe this one thing that Marriott should be really proud of being the only Hotel in the world that is apart of a Major League stadium.