SR1 ACTIVITY DIFFICULTY RATINGS [top most difficult]
Drug Trafficking - 1
Racing - 2
Snatch - 3
Escort - 4
Hitman - 5
Hijacking - 6
Tagging - 7
Chop Shop - 8
Mayhem - 9
Insurance Fraud - 10
CD Collection - 11
Demolition Derby - 12
Guide for the hardest activities [other than Racing because you just have to have a really fast car like the Attraizone and you need to get really good at driving, nitrous helps also.]
Drug Trafficking is by far the most difficult activity and LVL 6 through LVL 8 of these activities [excluding Vice Kings] are near impossible without exploiting various game mechanics or bugs. Judicious use of Forgive and Forget and picking up heavily armored vehicles at Rim Jobs garages are necessary to beat these. If you get close to the dealer's home base and your vehicle is damaged, you might be better off jumping out and sprinting to the blue marker. The weapon rewards from the Hitman activities don't really help with these activities because the weapons are randomly selected on activity start. The Titan [which I believe you unlock from Hijacking or you can just save a random spawned one] or the FBI SUV are going to be your best bet in terms of vehicles unless you can find a semi really quick.
Los Carnales Drug Trafficking LVL 6 through 8 guide:
This is hands down the most difficult one to complete without abusing various bugs and mechanics due to how many RPG wielding gangmembers spawn. The best method to beat it is to continually start the activity and if you do NOT have the .44 Shepherd revolver in your weapon wheel, instantly cancel the activity and restart. Additionally there is an exploit where LC pursuers will only spawn in the classic pickup trucks they drive, which are better for 2 reasons: Reason one is, they have less seats, ergo less gangmembers will get out of them at your stops and you are less likely to get RPG pursuers. Reason two is that the trucks are slower and less maneuverable than the typical LC cars which spawn, making the LCs less of a threat when pursuing you. I have been unable to ascertain why or how this truck glitch happens but if you continually restart the activity, you will eventually get this glitch. Essentially to make it easier, you want both random chances to line up, the .44 Shepherd in your inventory and the truck glitch. These lining up give you the best chance to beat these activity levels. Even with these exploits/bugs it will still be extremely difficult as RPGs can come out of nowhere and wreck you in one hit, so you will likely still need to use Forgive and Forget and pick up new vehicles [with high HP] at Rim Jobs. Check the map to see which drug stops are closeish to Forgive and Forgets and after the deal is complete, run [on foot] to the nearest F&F, the dealer will mostly keep up with you. It helps a LOT if you have infinite sprint for completing the Tagging activity. Try to keep the initial van that you get undamaged as possible for as long as possible, but once it is smoking, abandon it and run to the nearest Rim Jobs and get the Titan or an FBI SUV, save these prior to starting. They have the best HP of most suitable vehicles which can be saved. Without the .44 Shepherd in your weapon wheel, which is random on activity start, it will still be an uphill battle with all of these tips. You'll need full health items, "The Fist" from Freckle Bitch's is best. I basically only beat this one by the skin of my teeth, dealer was driving back to home base and our third or fourth FBI car was fucked, started flaming from taking too much damage and I just happened to be close enough that I could just sprint there and thankfully the dumb AI kept up and didn't get downed or stuck. Survived with just a sliver of health left too, this one was a total bitch.
Vice Kings Drug Trafficking Guide:
This one is actually fairly easy but the tips above will help you out with this one as well if you're having trouble, as far as I know the VKs almost never spawn with RPGs so you don't need to worry about that.
Westside Rollerz Drug Trafficking Guide:
Same as the LC guide, use the tips found in that guide but also be aware that in LVL 7-8 of this activity, there is a crazy long highway stretch to return back to the dealer's next stop and their home so be ready to switch vehicles A LOT. Infinite sprint helps here too so you can jump into your next vehicle. As a rule of thumb, the second your vehicle starts showing white smoke/steam, jump out and switch because while there is still HP remaining, in the higher levels of these DT activities the remaining HP will be gone extremely quickly and if your car catches fire at speed, it is very likely you'll die when you jump out or get run over by a pursuing vehicle.
Snatch Guide:
Essentially you're going to need a maneuverable four-door vehicle with good HP to beat these and it definitely helps if you have the weapon rewards from beating the Hitman activity. There is an exploit you can use to make these extremely easy. Call a taxi-cab from your phone menu and stand on the roof of it. Then, drop a grenade on the roof and immediately hit Y to get inside the cab, as it begins to drive away the taxi-cab will explode and the driver will get out. You can then exit the cab's destroyed wreck and get into the driver's seat. Since this vehicle is technically destroyed, it will take absolutely no damage from explosions, bullets or collisions and it's a decently fast four-door car for Snatch.
Racing Guide:
All I can say for this one is to get really good with the driving mechanics and get a fast car that handles very well, Attraizone is the one I used and the nitrous upgrade definitely helps.
Escort Guide:
The trick to this one is to focus on being elusive rather than trying to outrun the pursuing news vans or enemies, make as many sharp turns as possible, try to get the pursuers to crash into solid objects or other vehicles, run through parking lots and alleyways and other shortcuts. Generally speaking if you can stay on a wider/longer road you'll be able to outrun many of the pursuers but you still want to be elusive.
Insurance Fraud Guide:
Get a police car or an ambulance and repeatedly crash into other vehicles at high speed, you will be thrown through the windshield and launched out and in many cases, due to the 'civic vehicle bonus' you will very likely complete many of these levels with a single crash. This is definitely the most fun activity other than Mayhem, if you want to do it legit, go to a straight street and look for the cars which are speeding and weaving in and out of traffic or going around slower cars. You can get quite a high score just diving in front of these vehicles.
Mayhem Guide:
Fences. Run over fences with a car or shoot them with a weapon if you get bonus cash for doing so. It also works to just continuously run over lamp posts, mail boxes, benches, parking meters, traffic lights, etc. If you get a bonus for civilians you can just drive down the sidewalk repeatedly and it should be done quick.
Hitman Guide:
Some of these character models do not spawn in the area where the Hitman target list/menu says they do, so it can be extremely frustrating without looking up a guide. If you don't look up a guide, there's almost no way you'll be able to find most of these people given that they don't actually spawn. Roxanne does not spawn in the Apartments District, it's easier to find her in the High End Retail or Museum Districts. Juan also does not spawn in the Apartments District, and is often found in the Museum District. Tina does not spawn in the Barrio District and will often be found in Museum or Huntersfield. Many of the targets spawn in specific vehicles, if their outfit in the image looks like an ambulance driver, a meter maid, etc. it is extremely likely you will find them in their respective vehicles.
Tags are WAY harder to find than the CDs but both can be annoying especially if you don't keep track of which ones you've already gotten, thus necessitating you driving back and forth to every spot on the map once again.
I gave up on trying to unlock the 'secret stash' which is more of a easter egg and it doesn't count towards 100% completion, entering the safe code in reverse was just too fiddly and retarded for me.