Opinions on crests and their current and possible uses.
This post is less to talk about suggesting new crests and more to talk about existing ones, including which ones you absolutely adore and which ones you think can do better.
To give some ideas on current uses, here are some suggestions from someone on the official forums for at least the vast majority of current crests. Full credit to the OP of that post.
That being said, if there are current uses for crests that you discovered or you think are slept on feel free to share those too!
Here are some of my picks on ones I think can do better:
Leafeon: (It takes resisted damage from Fire and Flying attacks.)
I think adding to it's resistances was nice, but I would have liked if it also focused more on activating it's personal abilities more easily. Maybe attaching a built in drought to make it a sunny day setter. Admittingly that might be a little too good thanks to chlorophyll so maybe instead with the crest equipped it would automatically switch it's ability to mega sol. It wouldn't be able to take advantage of weather ball like meganium would because of it's low special attack, but it would gain permanent access to insta solar blade and have a boosted synthesis.
Glaceon: (It takes resisted damage from Rock and Fighting attacks.)
Similar to my Leafeon train of thought, I would make Glaceon a hail setter. It doesn't have slush rush so I think this one would be a little more feasible.
Magcargo: (It swaps Magcargo's Defense and Speed and increases Sp. Atk by 10%.)
I believe I understand the intent behind this one, it's supposed to become a bit of a glass cannon with this change. But even then it's just seems... mediocre. Not only is it's crest obtained fairly late into the game but it's base special attack is 90 and that extra 10% doesn't really do much to alleviate that. I think it would actually make a pretty decent choice between mid and late game if one, it's crest was obtained sooner and two, there was a shadow den option for this pokemon that learned eruption.
Infernape: (Infernape's Attacking and Defensive stats are swapped. 1/16th Max Hp is restored every turn.)
I hesitate to bring this one up, as I don't really have a suggestion to make it "better". The intent for Infernape's crest, as I have been lead to understand, is less to make it better and more to give it a 'side grade' option. Since base Infernape itself is already pretty good compared to it's other sinnoh starter counterparts. Essentially it's crest turns it into a speedy tank, which has a niche opportunity of potentially turning it into a scary set upper provided your opponent can't stop you. My point for bringing it up as, even with this crest option, there isn't much of a reason to pick Infernape over speed boost Blaziken as a starter. Though if someone has discovered a decent and consistent use for this combo, feel free to share!
I could share more, but I believe this gives a general idea. What are your thoughts? Any crests you feel could do better (within reason, of course)? And like I said, feel free to share crests and combos with them that you love using!