What is the most accuratey watch/HR monitor for HR zone training?

Optical is good but straps are normally a little more reliable. Training by heart rate I find really hard (I train with power now). The heart rate zones don't really work for me my heart rate pickups so fast trying to judge an effort is next to impossible to even hit a zone and maintain it. I personally think RPE is far better than watching heart rate unless all you plan to do is ride at steady state. LTP (lower threshold power) is the only one I can relate to heart rate and even then it's more of an interesting fact than useful.

I have had a Polar OH1 for a couple of years and love it. Never fancied wearing a chest strap and wrist watch sensors tend to suffer from poor contact so an OH1 on my forearm, paired with my Garmin 530 is perfect for racking up the vitality points.

I originally got my Polar OH1 in a bundle deal with a V650 which works out pretty good value with the vitality discount but the V650 is terrible for navigation so sold that after a bit and got a Garmin.

Polar chest straps (years) are way better than Garmin chest straps (months).

I use a Garmin 520 with HR chest strap, I find it reliable and use HR most of the time when training aerobically.

I personally wouldnt waste your time with a chest strap as the optical wrist and arm sensors are now so good.

Bear in mind you dont actually need good accuracy more than you need good consistency for training, as all you need to know is that your heart is in the same zone on each training session.

You seem to be looking at mostly watches - is that what you really want?  Bear in mind you need to consider where you display your HR data.   A watch makes a lousy cycle computer IMO coz its in the wrong place to see the screen.

If you want a new watch - get one, but be careful as the watches dont always integrate well as sensors to your cycle computer (if you have one).   Case in point - my lower end Garmin watch has to be put into "broadcast mode" to be used as an HR sensor and only 1 of my many (non-garmin) recieving devices reliably pics it up even though its meant to be standard bluetooth.

After messing around for ages I ended up with using the watch for watchy & basic excercise stuff and an optical arm band (tickr Fit) for HR training stuff.  I find there is a lot less fannying about with an arm band than a chest strap, because the order you put it on doesnt matter.  YMMV. 

I had poor experiences with the 'soft' Garmin HR monitors.  I had three of them, all failed in normal use.  The 'hard' one has lasted years, but not available any more. 

If you're looking at mainly heart rate training get the Forerunner 35 and a chest strap HR monitor. I like the Wahoo Tickr.