Best solvent all round is Carbitol (Diethylene Glycol mono Ethyl Ether), although this is not liked in some countries. For a non aqueous system, I would suggest iso Par, or possibly Dowanol. Carbitol will work with water based systems although, depending on the concentration of fragrance it may still need some solubiliser such as polysorbate 20. This may cause frothing. BHT is useful as a preservative.