Wow! Great choices. I've considered each of these and to me these are very different bags. Here's my two cents:
• LV Capucines - great quality all around. Very classic. Can be dressed up or down. Can hold quite a bit. Won't get dated easily. Will hold most of its value.
• Chanel Boy - my least favorite of the 4. For the price I feel they cut corners (fabric lining, etc. It doesn't hold as much as you'd think. Can be awkward to open / close. Will hold value but not as well as a Chanel classic flap.
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Gucci Dionysus - outstanding quality and design. This bag is well loved by many and still underrated. If you don't get one of the bedazzled appliqués I think you'll have a classic bag with staying power. I absolutely intend to get a Dionysus when the right color / material combo for me comes out.
• YSL Monogram Satchel - another gorgeous bag. Well made and stylish. Can be worn in a variety of settings. While I think it's classic in overall style, I don't know if the YSL on the front will date it. YSL has changed its way of monogramming a couple of times in the past and may do so again. While it will be slight, I think this bag will show wear the fastest of the four as its the least structured.
If price isn't an option I'd go for the LV Capucines or Gucci Dionysus. The Chanel Boy is nice but has never felt special enough to justify the price point. If I ever buy one I'd have to get a steal on it resale. The YSL is very nice but again not as stunning as the other three and will likely be the first to be discontinued of the four.
Let us know what you choose!