The one I reach for most often in summer (and a lot of other times, too) is my Madewell Transport Camera crossbody in sage green. It came with two straps, and I bought another, and I do like to switch them out, though I use the green and off-white stripe one 90% of the time. When I first got the bag, I thought it was a little too small, but then I started carrying a much smaller wallet, and it has become my favorite.
My older favorite summer bag is a Fossil Vickery, which I love because it has cream, taupe, and tan elements, so it looks matchy with a lot of summer shoes. I don't reach for this one as much anymore, because it is larger than what I've gravitated toward lately.
If I'm wearing my silver sandals, I'll grab my silver Kurt Geiger Kensington crossbody bag, which is also a year-round favorite. It hasn't held up as well as I would hope for; the lining has torn, he finish on the bag has worn somewhat, and the edges of the handle have some fraying threads. It is the perfect size, so it has gotten a lot of love.
The AllSaints Evaline crossbody that I bought at NAS last year has not become the workhorse that I anticipated. I think it's a kind of odd shape or something. Maybe it will come into its own, though, like the Madewell did for me.
If I need a big bag or want to take one on the town bike, I use my light gray Rebecca Minkoff Quilted Julian backpack, from NAS years ago. Still love it, though it's not especially handy for an everyday bag.