Adding a couple of drop-down menus to filter the Rankings I think would add a lot. A few that come to mind could be:

Time.        Filter by -> Week, Month, Year, Forever

Group.      Filter by -> World, Server, Guild, Friends

Difficulty.  Filter by -> Calamity level

Class.        Filter by -> Single Class, All Classes

Something like that. It's always nice to have a bit of a comparison. 

Yeah, a permanent ranking sounds cool, but, there aren't permanent rankings for anything else, is there?  Like Solare? if it's ever returning....  Why have ranking boards only for Boss Blitz?  In fact, they should revamp all bosses and include them in the Blitz.  And add some grind spots in LoML, making it livable like every other place.

My opinion.

One step at a time. IMO Boss Blitz is the second most reasonable place for rankings next to AOS (especially since they already put rankings in, just weekly instead of permanent). If PA sees positive results from a permanent leaderboard there would be hope for more ranking systems in the future maybe, but as I always try to say, focus on the most reasonable changes first. Again my big reason for the Boss Blitz rankings is to make use out of otherwise useless content (after receiving weekly rewards) 

We have permanent rankings for AoS. The season end rankings are recorded as snapshot and can be seen. 

As for this suggestion, given the fact that PA likes to change classes and gear and the bosses themselves sometimes, maybe a similar thing can be done here, where the "best of" are recorded on a seasonal level, so you can put things in relation (eg with maegu nerfs, previous lowest clear time will be much much harder to get close to after the nerfs, which can be demotivating). But in any case, yes to permanent. That way you actually know who manages to clear a certain stage, which is a big feat on the higher tier bosses.

In this case, a "season" is each week.  Just keep each week's results on record.

A week doesnt give much insight if youd have to check each week again. So finding a balance between "permanent" with its drawbacks (lack of comparability due to changes)  and "weekly reset" with its drawbacks (lack of comparability due to data being deleted) is necessary, but it should also be possible

Weekly: if people want credit for being on leaderboards, then they should continue to work at that.  If not, that's ok, credit for being ranked for the periods that they worked at it would be on record.  This also gives a chance for others who are actively playing that content a chance to be ranked (and a record of that to be kept).  So keep the weekly records (simply don't remove the data and provide a way to look back), and keep an all time list somewhere. Or, as was mentioned, like for grinding, it can be done outside of the game.

As for people putting hours and hours into it, and wanting credit for that - maybe have separate categories for number of tries it took.

I'm against that. 

An old record made with a class which got heavily nerfed later would be useless and clearly not representative.

Same goes for keeping weekly / monthly / season records, as time passes it will only get harder to track down the state of the class at that moment.

With a game which tweaks its classes every week, the only real record is the most recent one. 

i think the seasons idea works well with the games balance updates, just like with AOS