I often travel solo on Carnival but almost always booked a specific cabin except for the Spirit transcanal cruise last April. I booked a 1A guaranteed and ended up with a 4D. On Destiny class ships, some 1A with portholes are worth booking assigned as they are very nice and spaciaous, as long as you like cabins located in the bow.
I cruised on the Triumph, booked a 1A guarantee and that's what I ended up with. No regrets at all! It was a wonderful cabin, very large and with two porthole windows. I was glad that I didn't get upgraded, (unless of course it would have been to a balcony!) . Have you checked out the singles board here on Cruise Mates? Post your question there also, you might get some more feedback.
I had a 1a Outter on Victory and absolutely loved it!! I was in a 8a and other than having the Balcony, the 1a was way more spacious. You can also get it at a fairly reasonable rate. Portholes I loved as well. Only real negative about the room, is it is in the Bow so you do feel more movement there than ya do in other parts of the ship. I loved the movement, helped rock me to sleep at night ;-)))