“C’est toujours les meilleurs…” is a familiar form of speech which is appropriate to informal settings. In formal language you would say “Ce sont toujours les meilleurs” which has become quite formal(in a day to day use anyway).

As for the grammar issue, “C’était” as used here is an abbreviation of “Cela était” and not “Ceux-là étaient” which is never abbreviated in “c’étaient” to my knowledge.
It is again an informal but widespread form of speech usually less appropriate in writing.

There are a lot of “grammatical mistakes” like those that are deeply rooted in french and can be pretty confusing for learners.