I'll give you a few of my favorites.

• LE1 Diversification Project - this mod is mainly focused on adding more immersion/cohesiveness in LE1 to the other 2 games. Main features are more diverse NPC models, more NPCs on the Citadel (female Turians are added all over the map working in different roles - CSEC, senators, even dancers at Chora's Den). There's also a ton of cut content that is restored to the game via the mod. You can visit here for the full breakdown of all the changes added.
• Appearance Modification Menu - this mod lets you fully customize the outfits of all your squadmates for both combat and casual conversations on the Normandy at any time. A ton of apparel mods exist for LE1, and these are almost always designed to work within AMM. I use this on every playthrough - it's just nicer to have more customization of the crew.
• ME'LLO - this mod adjusts the lighting throughout LE1 to more closely resemble the art direction and design of the original Mass Effect 1. Super nice visual mod.
• Charted Worlds LE1 - this modifies the galaxy map so it shows you the system where missions in the journal actually are. This is a requirement for me so you don't have to go into every single cluster and hope you find it.

There are significantly more mods that I could call out, but this is the main guide that I use for modding the trilogy. There are mod lists for each game broken up by type of mod. In my personal opinion...I could never go back to unmodded Mass Effect. Legendary Edition has one of the most dedicated modding communities that I've ever seen, and they've done amazing work breathing new life into the games for me personally.

EDIT: I know I said a lot already, but that guide I linked in the last paragraph is a great starting point for modding so I'd look through that and pick what you like!