What is the correct way to say
Your suggestion is great! As it always is.
Your suggestion is great! As it is always.
Your suggestion is great! As always.
Your suggestion is great! They always are so.
Is any of the above variants correct?)
I would go with
Your suggestion is great, as always.
Your third line is also pretty good:
Your suggestion is great! As always.
Your first option could be fixed by saying
Your suggestion is great, as they always are.
or
Your suggestion is great! As they always are.
Although admittedly it is a little weird to mix "is" and "are" like that.
Presumably, if his suggestions are always good, he has made multiple suggestions in the past, so you should say that they "are" great. By contrast, if you say
Your suggestion is great! As it always is.
it sounds sort of like they have made the same suggestion multiple times, and every time they make that suggestion, that suggestion is great.
Your second line has the same problem, and even if it is fixed to
Your suggestion is great! As they are always.
It still feels a little unusual to me.
Your last line could be fixed if you wrote
Your suggestion is great! They always are.
It feels a little unusual to me when you say
Your suggestion is great! They always are so.