Just cast a love spell for this person to find a new more suitable mate. This situation comes up frequently, believe it or not, because some people just aren't comfortable (or feel safe) breaking things off with a partner. They'd rather the partner find someone new and move on.
I can't tell you which spells are the best or the strongest - it depends on the spell as written, the quality of the spellwork, the ingredients or techniques it employs, your strengths and abilities, and what ressonates with you on a deep level. Personally i like spells that use candles and herbs and are several days in duration. Others may prefer a fifteen-minute chant and visualization with no ingredients.