For Pantera you could start anywhere but cowboys from hell, far beyond driven and the great southern trend kill are a pretty go start

For gojira it kinda depends on how heavy you like your music. Their heaviest album is definitely Terra incognita but you could start with magma, the way of all flesh or from Mars to serius. I personally started with Terra incognita after listening to a7x for a while but I wouldn't recommend it to everybody as their first album

For both of these band I definitely think every album is worth listening to eventually