Which charge move is best for tyranitar for attacking? crunch? or stonedge?

Depends on opponent, but I feel that same typing for fast and charge moves is only going to get stronger, so I would lean towards B/C. Akso great against Alakazam and Gengar if you need to solo level 3 raids. B/SE is arguably a bettter generalist moveset.

I have both and prefer B/C because of the double dark moveset for psychic types. B/SE may do slightly more damage in general though.

For raw damage, B/SE.
Crunch is same type as Bite (dark), and is super effective to psychic types, which is better against Alakazam, and Mewtwo in future.
Crunch is also multi bar, which reduces the chance of overcharging the bar.

But what abou irontail? Got a IT/C which is rated best defensive moveset. Is it time to switch to bite?

I would think Stone Edge is overall better, unless you're fighting Psychics and Ghosts.

Crunch allows more use of charge move, and is narrowly resisted, mainly by fighting (its best counters) and dark (probably another Tyranitar). Strong fairy types are non-existent. Only drawback is the CD which is a bit on the long side for 3-bar charged moves.

Stone Edge on the other hand, while also weak to fighting, is resisted by steel (lesser probability to encounter strong steel attackers) and ground (which you would likely just use a water type counter), has far superior DPS and even resisted, still higher than Crunch with super effective.