depends? do you play paladin a lot? did you just see a streamers list running that card recently and now you wanna copy that deck? are there other "big" legendaries like zul'jin for example you are still missing?
it surely is a good card, OTK paladin also works without it though
Kangor is amazing in all paladin decks expect Odd Paladin. It synergies well with buffs, heals, Corpsetaker, Shirvallah etc. I would say it is second to only Tarim in the list of Paladin legendaries list.
The Glass Knight is also good but not as important as Kangor, many Even Paladin lists don't even run it.