Unfortunately, a lot depends on what you have/need to work on, as parts have evolved and new standards introduced, or proprietary parts that have come and gone that require special tools.
For me, the tool I use most is a park 3-way allen wrench (4, 5, & 6mm), but getting allens up to 8mm and down to 2mm is often helpful. Besides those, tire irons (plastic ones), a spoke wrench, a GOOD chaintool that fits your chain (or multiple widths) are pretty essential IMO. Others that help include, freewheel tool (for the freewheel/hub you have, chain whip (will work on any bike), whatever tools you need for your headset (depends on type, might just be allen wrench, but could require big/thin headset wrench), hub tools (old bikes would require the right cone wrenches), and I also use a cable cutter a lot to redo cables and housing.