Nick;
If you are talking HO, the BEST answer is none. Most smoke units in HO are very poor "substitutes" for a real locomotive's smoke. Most modelers in HO don't use them. The residue left on the locomotives, and surrounding scenery is very hard to clean off.
But if smoke is something you like, ENJOY IT! Whats important is to have fun, and if smoke is part of it, use it
Seuthe, of Germany, makes some of, if not the best, smoke units, so I would have to assume that their fluid has got to be the best as well.