We have a list of all known GSYNC/ULMB monitors here:
Blur Busters Official List of G-SYNC Monitors
Currently, unfortunately, there's no ULMB available in any 3440x1440 monitor, but hopefully this will change soon.
For the biggest ULMB monitors, your choices are 35"-sized 2560x1080 or a 27"-sized 2560x1440
All 3440x1440 monitors are VA-based, which usually ghosts a lot with dim colors, and they limit to 100Hz. If you are a huge disliker of ghosting, there is no 3440x1440 model that I currently recommend at this time unless you're willing to settle for a 75Hz FreeSync IPS model (
Acer Predator XR341CK,
Acer XR342CK,
LG 34UC98).
If you don't mind going on the FreeSync bandwagon instead of G-SYNC, and accepting more motion blur (simple motion blurring instead of ghosting -- see
LCD Motion Artifacts to compare ghosting with coronas with motion blurring)
One bonus is that IPS is available with ULMB at the 27" 2560x1440 size, making them cream-of-the-crop monitors:
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Acer Predator XB271HU
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AOC AG271QG
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ASUS ROG PG279Q
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Viewsonic XG2703-GS
There are TN versions of most of the same monitors above, too, in the same G-SYNC list. IPS has slightly more strobe crosstalk during ULMB than TN, but not objectionable amounts. IPS (usually) has better colors than TN.
Depending on the games you play -- for the immersive "ultrawide-replacing ghost-free" experience, you may also want to pair your 27" 2560x1440 ULMB monitor with a low-persistence VR headset such as an Oculus or Rift. They are particularly impressive if you're a person who's already loving in-your-face big-screen ultrawide gaming. Although VR headsets aren't necessarily very competitive-gaming friendly (for the current FPS type games).
That way, you have both options available:
...Whenever you play a competitive game like CS:GO or PUBG or RL, you use your 27" 1440p monitor.
...Whenever you want maximum immersiveness (big FOV) at low persistence, in certain kinds of games, you use your VR headset.
The best headsets already use a low-persistence mode that eliminates motion blurring & ghosting, so it looks practically CRT-like zero motion blur, although the resolution can be limited.