I still use Vanille's healing staff in the post-game content, since I use her as a healer primarily and the synthesis group bonus + improved cure are invaluable for me.

There is no singular best weapon in this game. Contrary to previous FF's there is no Masamune, there is no excalibur, etc. Each weapon and it's according third transformation / "ultimate form" has entirely different advantages and disadvantages and it comes down to how you use the character the most and how you build your teams. For example the Lionheart is an awesome weapon for the way I use Lightning (quick stagger causing, faster damage vs. higher damage), but isn't an ideal weapon for someone who'd use Lightning as more of a Commando where the Hauteclaire/Onyxsomething-or-other would be more ideal.

Stagger lock (that character can't trigger a stagger (However their summon can)) weapons are cool on Commando's - but severely hamper ravagers. There is no singular 'correct' answer here, it is entirely based on how you play the characters and how you build them and their primary roles in the team. So leveling one weapon up isn't exactly a waste - you can always use that weapon again later if you say, decide to use Light as a ravager instead of a commando primarily.

Generally speaking the weapon that the character starts with is the most balanced - it has equivalent Magic/Str values and no extra bonuses or negatives. Other weapons will have almost all Str and no magic and vice versa or lower stats overall and powerful secondary abilities or balance higher stats with negative side-effects.

An appropriately built character will almost always be stronger in one specific role than it's others even amongst it's three primary skill sets. I found it best to prioritize certain roles and then level the others up as a consequence of needing more stat boost and what secondaries you use the most.

I just maxed out the Blazefire saber (never transformed it into it's second stage) for Lightning until I got the Lionheart, which was more role specific and was the ideal weapon for how I use Light.