It is more important to reach 4* than 5* in most cases. In terms of damage, 4* weapons are going to be able to hold their own all the way to the Core. Some weapons benefit noticeably from being 5*, such as the Divine Avenger or the Brandish line which get additional charge attack power. The alchemers and catalyzers are also all much better weapons at 5*. The vortex bombs are practically useless until they reach 5* because they don't achieve their full capabilities till then. Blast bombs and vapor bombs benefit from larger areas at 5*, which is a big plus for any bomber.
Most armors/shields have negligible benefit from 4* to 5*. The popular armors, such as Vog/Skolver/Volcanic Demo get additional capabilites at 5* which makes them worthwhile to upgrade.
So what I recommend to everyone is to first get to full 4*. Then you want to upgrade in this order for 4* to 5*:
- Armors that gain additional abilities at 5*, including Barb Thorn Shield if you are a swordie and it is your primary shield
- Weapons that receive boosts in attack performance at 5* - DA, Brandishes, Alchemers, Bombs...
- Other armor
- Other weapons
- Shields
For T3 Lockdown, you really need all full-heated 5* gear, every bit counts there.
Question 2 - there are no top weapons, it depends on your personal play style. I would choose DA, GF, Ash... but that is just what works for me. These three things might not even be in someone else's top three.
1. Almost a given for armour, since most sets share the same precursors from 2-4*. The 5* is what you wish to use, out of the possible branches. Shields will always benefit from having more health and defence in their respective categories, with some shields gaining more bonuses. Dark thorn shield, when upgraded into a Barbarous Thorn and heated, has more attack bonus than its pre cursors.
2. Worth the effort for upgrading? DA, brandishes(maybe), alchemers, all bombs that are worth using.
DA gains 3 swords in the charge, massive increase from just the one as avenger. Brandishes now do even more explosions, so its worth upgrading. Alchemer charges at 4* is just 2 bullets from the original charge, at 5* it is 2 more bullets, for a total of 4. Bombs get increased radius which is always useful