Great question to ask because right now is a great time to use plugins in studio for building.
We have so many tools on Roblox and so many more tools in plugins that can help us out and it’s great that Roblox has that feature for us.
I would recommend the basic ones like:
Main
F3X - A building tool that provides a slick UI with similar features to regular studio but some features are altered to help development out. (Haven’t used this plugin for a while but when I used it you could use it live in game instead of just using it in studio.)
Studio Builder Suite - This plugin has a lot of features combined including resizing all objects on their own axis, setting your own axis on an object, rotating on first selected/selected axis and much more.
Extra
Tree Generator - This tool is very handy especially when you are doing a lot of work with trees.
Gap Fill - This tool is very nice for tedious jobs when you are trying to fill a crack and get precise placement.
Model Reflect - Model Reflect is a nice too when you need a mirror tool especially when you want perfect symmetry.
Rope/Curve Tool - This tool helps create nice ropes for jobs that normally would be hard to perfect or just take a long time.
Part to Terrain - This is a very nice tool especially when trying to convert triangle terrain into smooth terrain.
Part Counter - This tool helps figure out how many parts are in your game to keep your game as optimized as possible.
Atrazines Terrain - I use this one a lot and it helps quite a bit for custom terrain. You can place nodes anywhere and the UI is very clean and easy to use. You will understand how to use the plugin almost instantly. This plugin is to create triangle terrain.
Virus Scanner - A good plugin to make sure your game is safe.
ResizeAlign - Allows you to resize things easily and exactly on to corners for easy and accurate resizing.
Light Editor - Allows you to easy edit light inside of objects.
Edit 1: Fixed Links.
Edit 2: Checked and replaced malicious plugins with the true plugin.
Edit 3 - 3/19/19: Link fixes and new plugins!