Late to the party on this one but I just re-subbed recently and I love playing with mods so I'll give my input.
Bags: AdiBags if you like customization/filters and so forth, or LiteBag if you just want a simple 1-bag addon. ArkInventory is another customizable runner up choice.
Unit Frame: Shadowed Unit Frames has been mentioned already, I also like it. For the least amount of CPU usage you can use an addon to unlock the Blizz frames (like Leatrix Plus) and move them around instead.
Action Bars: Dominoes is another bar mod that uses less resources than Bartender since it's built more on Blizzard functions.
Raid/Party Unit frames: If you need more then Shadowed Unit Frames then try Grid2. (yes, Grid2 instead of Grid) Vuhdo and Healbot take more CPU to run so if you're tanking and not healing you may not want them.
Top/Bottom bar: These are usually referred to as "Lib Data Broker" or "LDB" mods I believe. TitanPanel is the sorta easy to use one with a lot of built in addons to it. Otherwise if you want to do it from scratch the Bazooka addon is the most lightweight of generic LDB addons, but then you will have to go out and download mods for each piece you want put on the bar such as a Clock/Gold Tracker/whatever. You can find LDB addons here
Data Broker : World of Warcraft AddOns or here
broker - Search - Curse
Damage Meter: Details! is lower on CPU usage than Skada, but uses more RAM which may or may not be an issue for you. I believe it is considered the more powerful addon between the two.
Chat mods/skins: BasicChatMods is an alternative that is more lightweight.
QOL mods:
TellMeWhen and/or AuraFrames are alternatives to Weak Auras if you find yourself not liking it as much.
GTFO - Audio indicator that you're standing in the fire and should move.
Quartz - Casting bar replacement
Scrap - Junk seller when you're at merchants
Scrap Cleaner - Works with Scrap to delete lowest value things from your bags when full (good if you farm a lot)
TipTop or TipTac - Tooltip replacement addons
BankItems or BagSync - Track inventory over alts
Postal and GnomishInboxShrinker - Mail mods