It really depends on personal taste. Americans will almost certainly be more familiar with large bowl-shaped all-seater stadia such as Hampden or Celtic Park, but there are plenty of Scots who prefer the old British style grounds with concrete terracing and old fashioned (read: "crumbling") facilities like Ayr United's Somerset Park or Albion Rovers' Cliftonhill Park. Personally, I like Dumbarton's Bet Butler Stadium for our home games as it's a comfortable, purpose-built modern facility, but I also like away games at stadia where I can stand on terracing or wander around the ground whilst the game is being played, such as Forfar's Station Park stadium or Stenhousemuir / East Stirling's Ochilview ground. "Best" is impossible to define as it is completely subjective.
Of the bigger, all seater, grounds, Tynecastle is my favourite. Of the traditional grounds, Gayfield (Arbtoath) is a great wee ground.
Tynecastle without a doubt. The stands are extremely close to the field so it makes for a great atmosphere. Hopefully Hearts will continue to be playing there in the future. Hampden Park - another great stadium that i was able to experience. I saw Scotland play Germany a few years ago in World Cup Qualifying and it was one of the best times I've ever had.
Somerset Park! Ok, slightly biased / tongue in cheek but there are reasons I like it. Close to the pitch with terracing. You were also free to move around the entire home end when I first started going so that you could view the game from behind the goal, corner flag or halfway line. I went to my first match at Somerset so standing / terracing became the norm for me. Sitting seems so alien & constricting at a football ground though that is partly due to the small space allotted to each seat. The seats themselves aren't the most comfortable but also leg room tends to be a little on the small side hence the constricted feeling. Are there any stadia with generous space so you don't feel like a sardine? Then there also comes the sit - stand to and fro during a match that comes with your team generating excitement / attacking play. Then if / when your team scores, everyone jumps up or around the place running the risk of falling over the seats in front of you due to how short they tend to be relative to you whilst standing. Having said all of this, I haven't actually been to a game in years so whilst there is the possibility of things having changed i'd doubt many of the aspects have or at least those aspects which would be a factor to me. Something I did learn about recently was rail seating / safe standing. Standing area that can be easily converted to seating when required. Maybe a good alternative.
I remember going to Glebe Park years and years ago. We parked the car in a field along with everyone else. Got out the car and there were sheep just wandering around between all the parked cars. Nearly hit one with my door getting out the car! Good times!
Another thing came to me re: Somerset. Sometimes you can hear the players talk / shout to each other. No always a good thing to be fair. I remember a Ayr v Celtic game. Probably a cup game but I forget which one precisely. Celtic had Dariusz Wdowczyk at full back (left if memory serves correctly) and heard his teammates call him, presumably by a given nickname due to the relative exotic first and surname..... "Shuggie"!
Tynecastle is an actually good stadium, but I do like Easter Road more...er, at least I did when Leith still was a bit rougher. It was part of the charm, and I loved looking north from Holyrood towards Easter Road in the evening.