I have to say I love the viability rankings list and think it looks a lot better than it ever has since I've started playing. If I had one change that I suspect a lot of people would disagree with, but I think I would change from a personal standpoint, it would be the following:
Glalie B -> B+
(I'll probably do Serperior B+ -> A- later...)
I will throw up my hands and admit it, I nominated Glalie down to B from B+. Today, I apologise. I knew nothing about the game, I was a fool. I did not know the power of moody or really understand the intricacy of Glalie beyond it being RNG.
Today, I'm tired of seeing people disrespecting my boy Glalie, it does not deserve to sit in the B tier. I'll try to address three points that I hear thrown around a lot when talking about Glalie:
1. Glalie is RNG based and is a luck based strategy
2. Glalie requires teams to be built around it
3. Glalie cannot put in work if the opponent has smart strike, sacred sword, curse, haze or taunt etc.
Glalie is kind of perceived as a gimmick by the community, but I don't think is anywhere near as luck based or as restrictive on teambuilding as has been stated in this thread. It is easy to just see the casino that is sub protect etc., however in reality Glalie is pretty consistent in the longer term in getting the boosts it wants. It will on average be +2.5 in every stat after 15 turns that sub/protect affords it, which will likely be longer due to either misses or inability to break substitute or its offensive moves being able to KO the opponent. In essence, unless you have a specific counter, Glalie is probably going to win. The problem I think is in the mindset of believing Glalie as close to Casino-gar, when it is actually closer to a set-up sweeper like Mega Salamence. If you let Mega Salamence get a free sub or DD and you don't have the necessary answer, it is probably going to win the game because you have allowed it the opportunity to set up. Glalie does, what Mega Salamence does, over a longer period of time. It is luck based if you get an evasion boost t1 and you dodge and that's why you win, but most Glalie games do not pan out this way. The Glalie player gets the boost 5-15 turns in after the opponent has given up positioning allowing set-up and then the opponent claims hax/luck and forfeits. If you leave glalie long enough, you probably will miss or fail to break a sub (or get parad). There is nothing lucky/hax about this, the probability of this happening is very high, this is a case of one player positioning better than the other.
Glalie can also work in a similar way to Mimikyu as a threatening 6th mon or as part of a second mode or as a check to fat. You can see two examples of this in teams that have gotten over 2100 this season:
https://piupoke.hatenadiary.jp/entry/2018/07/10/201418
http://yanko-poke.hatenablog.com/entry/2018/07/11/012535
In both of these builds, Glalie is actually used as a way to deal with fatter teams as a standalone pokemon. Fat and slower mons, specifically mons like Porygon2, are free set-up for Glalie. The first team does have spore breloom and the latter has twave mimikyu which both work well with Glalie, but both sets are also useful within the context of the team. Glalie is there as a compliment to the team to patch up weaknesses rather than as a centrepiece. Yes, you can have teams that specifically set up for Glalie and it certainly helps, however it is not always necessary.
One can say that Glalie does have counters that are common in the metagame. Lets use taunt tapu fini/tapu koko as an example of these. If you want to switch in tapu fini/tapu koko and taunt, if glalie subs on that turn, you give the glalie player two sheer cold chances, or a 49% to KO and then your glalie check is gone. Even then, getting +2 Special attack means that freeze-dry will 2hko fini/koko and without Z, neither KOs glalie. It is not so easy as to say this is a glalie solution. You can also switch out and come back in later. Same with scarf Kartana with smart strike/sacred sword. Kartana dies to a +2 freeze dry, so all you need to do is to sub/protect stall until you get those boosts. Mons like Toxapex can haze, but also can't break substitute and allow Glalie to fire off sheer colds or freeze drys for free as pex is forced to recover almost constantly to stay alive and you end up with a glalie behind a sub sooner or later. You could go through many other things like perish trap and say the same thing. Glalie always has a way out, that is the power of moody.
My point here is, you cannot simply switch these mons in on a subbed glalie and expect to come out mostly scot free. P2 can do that for mega mence, but no mon can do that for glalie near 100% of the time. You give glalie 1 turn and it can take an arm, especially if you have none of the things I mentioned above.
That's why I think it warrants B+.