My personal ranking:
1. Shard Bombs: With the right placement, you can dish out almost 600-700 damage per bomb. Knockback and blast radius of crystal bombs makes this bomb very difficult to maximize its damage.
2. Alchemers: Switch gunning proves to be the most OP DPS technique in the game if done right. Single-gun switching is a lot harder.
3. Cutter Swords: Requires the ability to plan your movements without diving into hazards. Could deal a lot of damage but you need to make sure all of your hits actually land
4. Needle gun: Powerful but you need to learn where to position yourself before you fire otherwise you're opening yourself to danger
5. Catalyzers: Similar to cutter swords; make sure you land ALL of your shots or else your damage is trash
6. Antigua: Better mobility than Needle Gun but easy to miss. Need to land all of your shots in order to maximize damage
7. Torto-fist Guns: Requires a bit of positioning but not too difficult to use. (I never used it myself)
8. Vortex Bombs: Small blast radius and requires a bit of positioning but nonetheless not too difficult to use.
Easiest Weapons:
1. Brandishes: Charge and hit. Out-damages all other weapons
2. Blaster: Just shoot. You can also utilize the switch gunning technique with Blasters but Alchemers have much higher damage output.
3. Pulsar: Bullet Streaming is the easiest technique in the game without having to reload
4. Blast Bombs: Place it and run. Good for speed runs
5. Haze Bombs: Just place it and wait for someone to walk into it. Low damage however.
6. Magnus: Charge and shoot, but requires more positioning than Brandishes.
7. Troika & Sealed Sword: Huge range and decent damage, but slow speed so requires a bit of timing.
8. Orbitgun
It seems like a pretty good ranking.
However, there are multiple ways to measure easiness. Take Antigua-line guns for example. In some ways they are easy (many fast bullets in regular attacks) and in other ways hard (charge attack locks you in place for a long time, low damage means longer fights and more opportunities to get hurt, etc.). And alchemers are not very hard, if you're not trying to get huge DPS.
I'd say that your ranking is something like: "For whichever has higher potential --- regular attacks or charge attacks --- how hard is it to reach the weapon's potential?" Let me know if you disagree.