I'll take whatever saw comes with a magic spell that prevents it from ever getting flooded, breaking a starter cord, losing the oiler, getting pinched, dulling the chain with 1/2 hour of daylight left, or any of the other hundred things that could happen if I were out cutting wood. You really need two saws.
Beyond that, I'd like a (working) crystal ball so I could know if in the next year I'm going to be scrounging little stuff here and there (in which case all I'd need is my 346xp), if I'm going to get more big stuff but nothing larger than 20" (in which case I'd keep the MS361), or if a friend is going to ask me to take away a 36" oak that just fell (which would mean the MS660 stays out, too bad about the weight).
Having to pick blind, I'd say 60cc would do me fine for 90% of the wood I encounter but I'd hedge up to 70cc just in case. I'd rather deal with a little extra weight than have to leave a big trunk because I couldn't pull enough bar to get through it. I guess if I had to pick one saw to keep and have the rest of mine locked away I'd make it the 372xp.
Good thing this is purely hypothetical.
Jack