Like the others have said: Answers will vary wildly based on the person, what their particular path was, what their desired path was, and what they want out of their career. Your mileage may vary, no warranty expressed or implied.
I was a 135 pilot, then 121 FO, now 91 corporate.
1. I'm already serious about good grades but which subjects should i work extra hard in?
Math and science, but realize that generic flying (i.e. airlines) requires little mental effort in terms of direct math-type stuff. It's a lot more situational awareness, critical thinking, and staying ahead of a multiple-ton, multiple-hundred-mph metal tube.
2. What is the best route to take after getting my high school diploma and what to major in. Aviation science in college? Or major in anything and go to flight school after? Or apply for Air Force and get everything at once?
Do not major in aviation. I did, now I'm about to start my last semester of my second bachelors degree. Realize that if something happens to you during your aviation career that ends up with you losing your job or being stuck outside of aviation - an aviation degree will help you basically nil.
3. What are some of your favorite things of the job?
Cliche - but flying airplanes is a blast. The people for the most part are good (I said most part). My particular job, as a part 91 corporate single pilot, is even more fun to me because I am in charge of everything. I fly the airplane, manage the airplane, take care of everything on the road, and generally make the show happen. I get all the risks and all the rewards.
4. Do you get healthcare benefits and discounts on flying?
I get full benefits, medical/vision/dental/retirement along with plenty of other random little perks (fuel bennies, gym membership, use of the airplane, lots of time off). At the airline I had considerably worse tangible benefits, but I also had access to non-rev travel, and jumpseating, both of which are huge perks and the things I do miss.
5. What is your personal favorite airline to work for?
I worked for a crappy 121 regional, so my opinion here is worthless. My favorite airline to work for is not an airline at all.
6. Is it a really stressful job?
Depends how you define stress. In terms of low-stress, day to day operations, yes there is a lot. Making the flights work, dealing with the headaches of passengers, ATC, ground crews, etc. can be stressful if you let it be. Once you realize that a lot of it is outside of your control, and all you can do is be the best pilot for your respective company, it really lowers your workload.
7. About how much do you invest in becoming a pilot?
Plan $65-75k to get yourself going (to the point of being a flight instructor). You'll have to build time up to at least 1200 hours to get a part 135 job (cargo, etc.). This is just flying expenses, and will vary depending on where you go.
8. Do you get to spend time with family and doing other hobbies?
I know my schedule far in advance, typically at minimum two weeks. I am away from home at most two to three nights a month. If I take people on anything longer than a three day trip, I'll either airline home or bring the plane back. I've only been called out twice for a trip, and both times it was something I had at least a little notice about.