When you ask which varnish is the best you will get as many answers and suggestions as there are brands. Just add this to the list and then you will have to make up your own mind.
When people ask me if they should varnish or not, I always respond that if you don't, the time will come when so much of the soft grain of the wood wears away that you will want to do something. This may be too late depending on the depth of the errosion. Sanding might remove the plugs. So I suggest to do something right away and if you intend to varnish you should commit yourself to doing it right. That is, a minimum of 10 coats and an additional coat every 6 months. If not, paint or let it go natural.
The polysulfide is another problem. Some products have some silicone in it and if it does the varnish will not adhere to it (after a year or so). Also, polysulfide is more flexible so it will move more than the teak. For this reason, avoid using any two part varnish that is too hard because it will crack and will be difficult to repair so I fully agree with John and his comment. So that narrows it down to the single part varnishes with tung oil. They are all good but some are easier to work with and leave a better finish than others.
Now....my own personal opinion.... There used to be a varnish made by Epifane that permitted coats to be applied withing 48 hours without sanding. This is a quick way to build up but it does not have as much UV protection as other varnishes. I used this varnish on my own boat for the first 8 to 10 coats. Then a real good sand with 180 grit. Then I really like the finish of Goldspar and Captains. Goldspar has a little more pigment in it so it gives it that golden color, while Capatains is clearer. They are both excellent varnishes as are other brands. I have not tried them all......
If you intend to varnish where there is no polysulfide, you may want to consider a durable, hard two part varnish like Honey Teak as the first 3 + coats then varnish over it. They say not to but that is BS. As for Nereus, she has about 12 coats on all her exterior woodwork and I give her a coat every 6 months when she is in use. Cover the teak when not in use.
The bottom line is it is a lot of work and unless you are committed to doing it right then use Don's suggestion of varnishing or painting the sides and let it go natural. One last comment. If you go to all this work, don't let it go and have to start over again.