First up, one I have, despite not having a forum account, been waiting for several years for (and is finally in open beta for tModloader), Scooterboot9697's Metroid Mod!
Next, the incredible (but currently WIP) mod, Starlight River!
Last one (for now), is Upgraded Accessories! This adds in combinations of accessories, and a few new ones. It's even compatible with Thorium if that's your type of thing!
I personally use a combination of The Soft Pack and Better Ambiance. Rather than changing the visuals radically via either completely different textures or high-res ones, these simply give the game a visual facelift.