Here are two that I commonly use. For maximum smoke, go with the 20 ohm, if you want to go a bit more conservative, go with the 22 ohm. For diesel upgrades, I usually stick with the 27 ohm factory stock resistor as diesels shouldn't pour out smoke like a steamer.
For either of these, I use my Dremel wire wheel to lightly knock off the powdered ceramic coating. Run the wheel across the resistor in line with the winding or it'll likely break the nichrome wire windings. With a gentle hand, I've done at least 50 of these with only one casualty, that was early in the process where I got a little careless with the direction of the wheel.
Both of these are for standard TMCC smoke outputs WITHOUT the smoke regulator. If your locomotive has a smoke regulator, stick with the factory supplied smoke resistor. The one exception for the smoke regulator equipped locomotives is if you have a 6 ohm resistor, I'd recommend changing it to an 8 ohm resistor. The 6 ohm ones tended to take out the smoke regulator module a bit too often.
