Recommending FCOM to a first time mod user that's already overwhelmed by choices? :facepalm:
Save that for later.
If you want to try an overhaul mod, I'd start with either
OOO or
TIE.
Other mods I'd recommend:
OBSE - Required for many of the mods listed here. Not actually a mod.
Unofficial Oblivion Patch - Fixes bugs
Unofficial Shivering Isles Patch (assuming you have SI as well) - Fixes bugs
Unofficial Official Mods Patch (assuming you have the DLCs) - Fixes bugs
Toggleable Quantity Prompt - Updated - Makes buying/selling/dropping items more convenient.
Cobl - Doesn't do too much on its own, but required by many mods.
Enhanced Economy - Makes buying/selling more realistic
Harvest [Flora] - There's a newer version than what is bundled with OOO. Allows you to see what's been harvested.
DarN UI - Makes the user interface less console-y.
The Ayleid Steps - More dungeons to explore
Super Hotkeys - Adds more customizable hotkeys. There are other hotkey mods that you may prefer to this one.
RAEVWD - Makes more objects visible from a distance. May not want to use it if your system can't handle it.
QTP3 Redimized - Makes the game look better. There are other texture mods that you may prefer.
Map Marker Overhaul - Allows you to manage the map markers.
Display Stats - Shows extended information about items in your inventory
Custom Spell Icons - Allows spells to have unique spell icons instead of sharing the same icon per school.
ALEX - Adds hidden chests / items to ayleid ruins. A treasure hunt of sorts.
Mountain Tower - A home. Plenty of other homes to choose from, but this is one of my favorites.
Better Book Collision - Makes books less likely to fly out of bookcases.
Book Jackets - Makes books look better and more unique.
TNR - Makes npc faces look more varied. Some don't like it, but many do.
Any body mod - Many to choose from, down to personal taste. Makes males / females look nicer.
Keychain - Keeps keys from cluttering your inventory.
Art of Speedy Depositing - Makes it easier to stash items in containers by automatically moving them based on your preferences.
Better Letters - Adds a little flavor to the game by adding text to blank notes.
Gather ye Rosebuds - Makes harvesting ingredients easier.
VAs Better Gold - Makes septims look better.
MiniMap - Adds a minimap to the game.
Glory of Cyrodiil - Replaces the startup background to something a little more interesting.
That should give you a good starting place. If you don't care for one of the mods listed, but like its concept, you can always look for a similar mod. Most of these mods have a number of possible alternatives.