Apologies in advance for length.
I don't know, I think there's a good ending. It's just my opinion, but I liked synthesis. I'm not saying it was executed all that logically, but I think the implications make it one of the stronger endings in any game I've played. Admittedly, I am a bit biased because I am getting my degree in Sociology, so implications like those involved in synthesis REALLY excite my passion for the subject. (I actually wrote a paper in my undergrad based on this)
Synthesis for me represented the only true escape from the "human condition." Look at our world today; it's hateful, cynical, often uneducated, and often brutal. That these qualities exist even in Mass Effect's far future societies tells how prevalent they STILL are. Now imagine an outcome (in a game, I know, but this is that passion of mine) that could produce not just a world, but a UNIVERSE without racism, sexism, age prejudice, sexual-orientation prejudice, class prejudice, and all the other shortcomings present in EVERY societal existence! A universe with not only the collected knowledge of every species we have ever known, but also of the billions that we haven't! The barriers between superiority and inferiority truly vanish for the first time, and the potential of EVERY individual transcends all precedence! The way is open for true objectivity, access to an unending reservoir of knowledge, and the founding of a true Utopia on a galaxy wide scale! Think of how inconceivable these ideals are if you try to reconcile them in our society today. Achieving these outcomes was what made me hobble my Shep for that green beam.
I didn't like the other solutions for a couple reasons. First, Destroy reminded me of the ending in Assassin's Creed III. Yes, I destroy the Reapers, but there's still no hope for lasting change in such a solution. It's a Band-Aid on a bullet wound; history will continue to simply repeat itself. Sooner or later organics would make more synthetics and repeat the same Terminator cycle we always see. The social implications don't change very much either, and would likely only get worse with the scarce resources (We gain no new knowledge to repair mass relays. No more relays = no more trade.)
I think Control is the least moral of all the options. It skips ALL notion of elected officials or any sense of self-government or self-determination. Grand Master Shepard elevates his/her self and gets to make completely arbitrary decisions with the most powerful military force in the galaxy on a whim. There's ZERO influence from any sort of democratic process from ANY species, and the galaxy gets to bow down and smile. I don't see what authority he/she has to turn the galaxy into a perpetual dictatorship. A human who's species has been spacefaring for a couple decades gets to now dictate to civilizations who have been spaceflight-capable cultures for thousands of years???
I completely understand the strong argument as to what moral authority Shepard has to re-write others on a genetic level. However, as I see it, it is the "least morally wrong" choice of all the options. To choose destroy is to (eventually) sentence the galaxy to more conflict between organics & synthetics. Every death caused from such conflict is in a way on Shepard's head for failing to advance societies past such conflicts right there and then. I've said my piece on control, and I just think that for the benefits synthesis grants, the moral downside is outweighed by the greater good. I think synthesis's moral dilemma also show the cost a society would have to pay in order to achieve a perfection that we cannot know in our present forms, and that's a deep concept I don't see echoed in the other endings.