I think most bicycle mechanics will agree that when it comes to tool selection there are a few tools that you just couldn’t be without.
Personally the 4 and 5mm T-handle allen wrench are my most commonly use tool, I wear them out frequently and I only buy these tools if they have a lifetime warranty like Craftsman or snap-on. I have both craftsman and snap on t-handles allens and I prefer the Craftsman because they are a bit shorter and can fit into tighter spots on the bicycle.
Next up (and this is a 2 way tie that I will count as one tool) would be the park tool three way allen and socket wrench for their versatility, convenience, and back pocket feel. They are time savers, the less time you spend reaching for a tool the more productive you can be.
Cassette and freewheel removal tool there are a tone of different versions of this tool I use the park tool FR-1 and the FR-5G these are the budget tools. If i'm looking to save time I use the Abbey bike tools dual sided crombie tool its designed so that you don’t need to remove the quick release from the wheel. Using the crombie tool and the next on the list tool Pedros chain whip you can have a cassette removed in seconds.
The padros vice whip by zinn is the best tool andeveryone on the planet should own one. The vice whip makes removing the cassette fast and no flex like you get from a park chain whip.
A high quality adjustable wrench in 3-4 sizes should be in your shop or garage at all times. They are needed often and the adjustability makes invaluable, also if you need to hammer something you can use the back side in a pinch.
3 way spoke wrench for tensioning and truing wheels of various shapes and sizes is a mush have to keep wheels running straight.
As far as a tool you don’t really need but makes your life a lot easier go with a park truing stand . if you don’t want to drop $250 on a professional stand find a zip tie and zip it to the chainstay, clip it short with side cutters, and boom truing stand.
Lastly I like to keep my tools with me and transport the often and for that I use a high quality tool roll like the 32 pocket tool roll organizer by rugged tool. This tool roll helps me keep my work space organized and efficient.