there's no "best WP theme" because each theme is different and most of the good ones do the same thing anyway. So, it's hard to differentiate one from the other. Besides, google makes anti-SEO updates more than I change my socks (and I change them daily), so many themes that are good at some point eventually get abandoned because of google's craziness. And those who keep getting updates are pretty known and always on top, and are probably themes like Avada, Genesis, and many others.
Personally, I use Weaver Xtreme and I highly recommend it to everyone. It's free (you can install it from within the WP repository) and it's as optimized as any other premium theme out there but the downside is that it's a beast in its own right. It will take you few days of customization to make it look and behave the way you want it to. And you also need to know a bit of HTML and CSS in order to adjust the sections for which there's no toggable option in the theme's customizer. So, many people (especially beginners) stay the fuck away from this theme, but like I said, personally it's what I use and can recommend.
I used Avada in the past, too and while it can be customized to the same level as Weaver or other premium themes I found Avada to be way too complicated for me.
But anyway, if I ponder your statement better (the "SEO optimized" part of it namely) I don't think there's a single theme that can be "SEO optimized". I mean, OK, the theme should load lightning fast and many premium themes load like that, especially when your website is small and you have few plugins installed. But other than speed I don't know what else is there about an SEO optimized theme. Eventually, every WP user ends up with SEO plugins to compensate for a theme's minuses.
What I can recommend is you take a look at the various big WP theme providers on the market today. There's a guy here on the forum -
@Festinger - whose entire business is WP, so you can find anything WP related in his vault. I have never used his products yet but there are many members recommending him so he's probably worth something
And if he can't help you, then there are Envato Elements, MyThemeShop, CSSIgniter, and many other WP theme providers, and their premium themes will definitely satisfy your needs.
And if none of what I've said is still helpful to you... well, just run a google search for "the best SEO optimized theme for Wordpress" and you'll find plenty of good recommendations... hopefully....