Formal:
* Isso é lindo. (This is beautiful)
* Aquilo é lindo. (That is beautiful)
* Essa é linda. (That one is beautiful - feminine)
* Esse é lindo. (That one is beautiful - masculine)
Informal:
* Que lindo! (How beautiful!)
* Isso é bonito! (This is beautiful!)
* Aquilo é bonito! (That is beautiful!)
* Essa é bonita! (That one is beautiful - feminine)
* Esse é bonito! (That one is beautiful - masculine)
Other options:
* Isso é maravilhoso. (This is marvelous)
* Aquilo é magnífico. (That is magnificent)
Remember:
* Isso and essa are used for things that are close to the speaker.
* Aquilo and esse are used for things that are further away from the speaker.
* Use linda for feminine nouns and lindo for masculine nouns.
The best way to say it will depend on the situation and your relationship with the person you are talking to.