Share some useful plugins that you think are very important, but hard to find.
Thanks for asking and my pleasure to answer your question.
Now WordPress is not only a blogging system its convertible, but it can also be used as an online store, a blog with the store, referrals website with a store and blog, project management, or client management system, etc..
So first we need to define the purpose of the system what it needs to achieve to define and list down all the plugins.
Second things there is no such a thing called hard to find ofc. there is but most of the companies they build their plugins and they hire marketing teams to be well known if it's not known might be built by a shady company what might be having a question mark. Or unless if the plugin built and mean to be hidden for in house use only.
So first we need to define the system what will have then we can list down all the features. In general, the most important features are:
SEO, Security, Caching plugins or load enhancements plugins, page builder, mobility
There is a ton of plugs for social media management, auto-posting, content creation, GDPR, etc.. you can simply search for them ill not explain those so if you have any specific question please ask and would be happy to answer
Now it's important to understand that plugin as they are sharing the same infrastructure and if the plugin is not well written under WordPress community guideline, it might affect the speed of the site or the security so here are my best plugins that I can recommend for each feature in above.
SEO:
SEO Auto Linker is a WordPress plugin that takes care of all of your Internal Linking needs.
Yoast plugin Premium if the free been used it's useless alone and it must be combined with other plugins
There are people they like to add broken link checker and keyword research same as you are putting your bedroom above your small car those extra plugins might generate load on the site what another professional external tool can do better work.
one My favorites for SEO to be honest are is Rank Math or Premium SEO Pack but there is a learning curve especially for the second one
Security:
iThemes premium (Free missing a lot of important features if you need to use free you can go with all security plugins), Wordfence (much better to test the load of the site before and after as some hosting work bad with it), Sucuri plugin
Caching plugins and speed:
WRocket (The best plugin it's not free I have the nulled version if you need it I can post it in download section). W3 Cach, a total cache is free they not that efficient but better than nothing
Gonzales Plugin
Swift Performance
Page builder:
This is an important part now there are many options here now it's important to understand there are many plugins that they embed codes that are not a necessary example might some plugin add the accordion element in all pages what will cause a lot of time load (same what happen in wp bakery) here where Gonzales plugin comes to work. so, in my opinion, the best plugin builder is beaver builder plugin it's so expensive but do a lot of great work I have it if you need me to share it as well ask for it if you like to try it.
The best builder plugin I found from a performance perspective is the oxygen plugin but it doesn't include all visual elements
Mobility to make it fit all the screens:
AMP for WordPress but this is not what I prefer as I build specific templates for mobile
Wp bakery plugin has an incredible external plugin I think it includes all visual elements you need in the world to embed in the builder and so easy to use but downside it embeds a ton of feature and not necessary codes in the page where it slows the page down...
I am sorry I went into so many details but asking about the important plugin its really a general question as WordPress can do many things so if you feel you still need to ask more please feel free to ask more about anything you like. and would be happy to answer all your questions.
How can I convert PSD to Wordpress...Do I have to make an HTML first
There are many tools now for this Artisteer or photoshop plugin like the export kit. When first time I work with Artisteer it was like a dream tool it short days of works into seconds, an Export kit plugin you just need to make the guideline and this tool will export to everything for you but what I prefer is to do manually
Those tools are the best for the time perspective but the embed a lot of the trash in the code what slows down the page (maybe) I prefer if I need it to be professional to render it to html5 using Macaw or InVision now then convert this to PHP template files also beside converting those to WordPress theme they need a lot of testing what makes the first 2 options are the best for timing but might be not the best in quality so you need to check that.
Export kit or any others will convert everything to the theme what you don't need to render to HTML5... Hope this answer your questions if you feel you have more to ask please go on.