Glalie isn't the best.
From Smogon: Glalie is mostly outclassed as a Spiker. Bad typing, mediocre stats, and an inconvenient matchup against common leads do not help it at all. However, it does have a niche, and that is being a decent suicide lead. With Taunt, Spikes, and Explosion, Glalie can often set up several layers of Spikes while preventing opposing entry hazards and is often useful for offensive teams.
From Serebii: Glalie is an unexceptional Pokémon. It has all-round Base Stats of 80,
making it acceptable yet forgettable in pretty much all areas. It's
capable of defence or offence, but it's mediocre at both. Being a pure
Ice type gives it a nice offensive STAB, but defensively it leaves it
with a single resistance and with several weaknesses. Its move-pool
has some possibilities though. In particular, its physical arsenal
contains the offensive staple, Earthquake, and the ever useful,
Glalie's best use in its tier, NU, is setting up Spikes with Taunt. It outspeeds most of other leads (albeit Scolipede) and can use Explosion to limit the other lead's entry hazards. Movesets are on the links above.
Source: Above Links