Regarding 1h hammer vs 2h hammer situation.
Currently 1h hammer + Raw Dragon Scale outperforms 2h hammer in every scenario.
The following tests were conducted with two t15 weapons (thanks to Madman).
No relationship buffs, which would push the outcome even more in favor of 1h.
Ch/S
Blood Arrow, dmgfood, Pet, double titlesystem (9% dmg, -attackspeed)
1h: 7.801.045 RP
2h: 7.771.481 Rune Pulse
fullbuffed
1h: 43.229.924 RP
2h: 41.203.677 RP
Keep in mind, all Ch/S ranged skills profit from the 13,44% more damage from talisman, which makes 1h + tali vastly better than 2h.
Ch/M
Power from Within, dmgfood, Pet, double titlesystem(hammer dmg)
1h: 11.029.404 RP
2h: 11.038.737 RP
1h would actually be better than 2h, even in offburst, since I didn't have a %dmg relationship buff.
fullbuffed WITHOUT Elemental Rampage (which increases 1h dmg only)
1h: 93.769.672 RP
2h: 85.751.898 RP
As you can see, quite the difference in burst, even without rampage, since Ch/M has many % damage buffs.
Ch/Wl
Selfbuffs + dmgfood, 7 forgestacks double system(hammer dmg)
1h: 16.214.036 RP
2h: 16.081.244 RP
fullbuffed
1h: 107.518.575
2h: 101.721.561
Haven't tested /Druid and /Priest explicitly, but judging from their buffs we can safely assume 1h outperforms 2h as well.
Since the aforementioned classes won't reach the attackspeed cap either way, there is no drawback to using a 1hander. Quite on the contrary, you get more frequent Shock Overload procs, faster forge stacks, more Chain Drive procs and more patk (offhand).
I don't play R/Ch myself, but I have been told that 1hammer + wb dagger outperforms 2h as well.
The superiority of 1h stems mainly from the much better scaling of faster weapons (that have lower pdmg) with damage nonstats from gear, cenedril buff & robot and their interaction with % damage buffs. The new wb talisman made it even worse.
I'm afraid it is no exaggeration to say that 2h hammers have become useless. To my knowledge, there is no other weapon that isn't best in slot for at least one single class combination. Even 2h axes have their use on Druid/Warrior.
Please make 2h hammer a viable weapon again. Two possible changes that came to my mind:
a) a significantly better %2h hammer damage title for the challenge slot
b) increase Finishing hammer's 2h hammer damage increment
tl;dr: 1h + talisman outperfoms 2h hammer in every way on all Champions, make 2h hammer viable again