Discuss and learn French: French vocabulary, French grammar, French culture etc.
Well I think you either say "A mon avis..." or "Je pense que....." since both mean the same thing (in my opinion-I may be being pedantic).
A mon avis (je pense que ) les maths est interessant is incorrect since "les maths" is in the plural.
Both of your other alternatives seem acceptable to me.
Hoping that I am correct...