There's no singular most important information on lab results bud, pretty much everything that's tested for can be seriously problematic if out of range. Stan Efferding recently (well kinda recently) went on Mark Bell's podcast and mentioned that their particular service they shill for (Marek) is offering a panel that checks out what they believe to be the most essential values for health/longevity, I'd start there if you don't want to pay to have every little thing looked at. Look around on Mark Bell/Supertraining youtube channel for the info, it should be sometime around the beginning of this year. Bloods are not your only line of defense against potentially life threatening issues, though.
Biggest thing you can check yourself often is blood pressure, I highly recommend you try to check yours at least once a week. High blood pressure requires no blood draw or lab to test for and it's incredibly dangerous to let go unchecked. You can also keep note of the color of your urine and pay attention to your poo because signs of liver and kidney issue will show up in these two and that also requires no testing to identify the beginning signs of organ trouble. Keep note of your fingernails as the health of your nails can provide insight on various internal issues, especially if they begin yellowing.
I guess my personal answer for #1 thing to look at as a generalization is blood pressure. I know most users do not get testing done ever and I understand that people have various reasons for not getting them but if there's one purchase I'd advise everyone to make it's a blood pressure monitor. Could seriously be the best purchase you ever make, literally could save your life.