First, I would ask whether you are actually being asked which philosopher's ideas you "like" or which you find "convincing". The first implies an emotional reaction on the order of "I like Spinoza because he reminds me of my uncle" and the second an intellectual evaluation. If your assignment actually used the term "like", then a good opening might be to discuss precisely what it means to "like" a philosopher, as opposed "consider their arguments valid" or some other reasoned judgement.

If you are being asked which theory you find most convincing, you should open your essay by outlining the criteria on which you are making your judgements. Is there some area you consider one philosopher to have addressed that others do not cover? A phenomenon you have observed or experience that you have had that coheres better with one system than another?