Suppose you are now entering a ラーメン屋. The restaurant have three kind of ラーメン as "醤油ラーメン、トンコツラーメン and 塩ラーメン."
Case-1: You are the party of more than two. Others ordered "私はトンコツ！," "私もトンコツ," and you can say naturally "私は塩ラーメンがいい."
As you know "私は" emphasise that "ほかの人とは異なり、私は."
"私は" implies the existnce of the other's choices or situations and "My choice" or "My situation" is diferent from others'.
Case-2: You are alone but the perosn next to you made the order of "トンコツ" and you and the waiter (or waitress) heard it. Then you make order saying "私は塩ラーメンがいい." You don't need to take care of others' choice but it a little bit can make sense emphasising that my order is different from my next gest's order ("Never confuse them").
Case-3: You are alone and the first guest of the restaurant and make order saying "私は塩ラーメンがいい." It's also OK!
If the ラーメン屋 is very famous for its トンコツラーメン, it can make sense.
I know that everybody choose "トンコツラーメン," but I myself choose "塩ラーメン!"
If it is even three stars restaurant you can say "私は何々がいい." This expression to indicate the order can be used any restaurants in Japan.
To be more polite you can say "私は何々がいいです" like "私はミディアムレアのステーキがいいです。"