Most painful problems: Mostly Apple hardware repair. Ever tried to replace the bare LCD panel in a Macbook Air? My advice is, DON'T. Buy the whole upper assembly and save yourself the hassle. Apple people are willing to pay, so don't be afraid to do things you'd never get away with with PC's. AIO PC's (including the iMac) absolutely SUCK! If the client has any hardware problem other than RAM or HDD, I get them to trade it in for a traditional tower/monitor setup. Gaming laptops (ASUS, MSI, Alienware, etc.) can be a big pain too. Especially when the gamers that own them expect them to be cheap and easy to upgrade like a desktop. Again, if it has any sort of major hardware problem, I try to get them into a gaming desktop and a nice business class laptop if they need portability as well.
Most profitable: Hard drive replacements and/or SSD upgrades. Seriously when you factor in the markup for the new SSD, transferring all their data and such over, and reinstalling software, you've got a good few $100's worth in profit. It's easy as pie too. Replacing Apple laptop screens is easy and profitable too. Like I said before, Apple people are willing to pay through the nose. So long as I'm cheaper than Apple (not hard to do), they're happy. I just replaced the LCD on a Macbook pro for $400. My cost was under $100. I purchased the entire display assembly for $90 and had it installed in 5 minutes. Apple quoted them $600, so they were happy to pay $400.
Least profitable: Smartphone and tablet repairs (excluding Apple devices). iPhone and iPad repair is easy as pie and is a GREAT revenue generator. However most other tablets and smartphones are a real PITA. My advice is, hire a dedicated tech to take care of the "device" BS and put your efforts to better use. If you're doing it yourself, focus on Apple device repair only. It has the best markup, the parts are easy to find, and you won't be wasting 2 hours trying to fix a $70 throwaway tablet that the client probably isn't going to pick up when it's done anyway. Virus removals also suck. They're time consuming and not very profitable. Getting $80 for 2 hours of work is terrible. And EVERY problem the client has from there on out will be YOUR fault. "It worked great before I took it to you to remove the virus! Now my printer won't work!" Or they blame you when they reinfect themselves. I do everything in my power to prevent my clients from getting infected. I hate doing virus removals.