Assuming you're asking about vegetable gardening rather than flowers? I've read mixed opinions about wood types, mostly related to leaching of bad things into the soil. I'd leave the chemistry part to the experts.

I once built raised vegetable beds from redwood fencing I tore down, 1x10s or something. The stain had been weathered some and it never visibly changed much over the years. Three widths tall, they tended to bow out with the weight of the soil within, even with reinforcement. But being able to condition the soil was the best part, and the production of the garden was terrific - root crops, tomatoes, everything did nicely.
Next project (different place) used untreated 8x8s, cedar I think, and was a construction behemoth - drilled and spiked them together. Three boxes 4'x8 held everything we needed to plant. But they looked and performed great and lasted forever - again, soil depth and conditioning was the key. In both cases they were open-bottom beds, and I dug them out about a foot and turned them over real well before filling them with a mix of black dirt, sand, and compost. I'm sure there's great books out there on this topic- look up 'square foot gardening' as a compact alternative (not very raised, however if you're trying to solve a rocky soil problem). My 4x8s were also three layers tall, but it did not deter momma bunny from dropping a litter under the mulch in my onion rows one summer. Large-footprint crops like corn & potatoes did better in a conventional plot. Don't discount wide-beam containers providing a spot to sit while futzing in the garden. Good luck.

I used cedar for my first raised beds. When I took them apart years later there was some rotting but not enough to need to be replaced.

Now I have two large raised beds made from extra cement landscaping blocks 8X8" standing on end, three across each way. I dug down and placed them. After five years they started to get out of line a bit (see corner) so last year I dug down and pushed them back in place.

I watched a home show that used pressure treated wood saying that arsenic is no longer used in the wood.

If you Google raised beds you will find lots of suggestions.

A few suggestions. The square foot garden method uses raised beds. They can be as long as you want, but only 4 feet wide, so you can reach in without stepping anywhere. They fill the bed with 1/3 compost, 1/3 vermiculite and 1/3 peat moss. I sort of follow this method, but lean heavier on the compost. You can purchase the book or get it at the library. I made my beds out of normal 2X6 pine, using the directions in the link below. I didn't treat them and they're going on their 4th year. It is 12 inches high. Don't put anything on the bottom, just put newspaper or cardboard to smother the grass, and fill it with your soil mix of choice.

Here is a link that might be useful: Sunset.com raised bed

Well there are many choices. I made mine out of cedar because that's what my wife wanted. :)

Cedar looks very nice, but is very expensive.

Jon Hughes swears by his cement block beds. They don't look great, but they are very functional. Jon loves to brag and has videos on youtube on how he built them and what he put in them. Here is one such post.

Here is a link that might be useful: Jon Hughes beds

I am a scrounger and made my raised beds from some cedar siding that my neighbor had left over. This was last year so I can't tell how long they will last.

We made 3 raised beds using recycled decking. My co-worker and my BIL both had decks they were tearing down. They both were at least 20 years old. We had tons of 2 x 6x 12' boards. The tallest is 30" tall. They were built on a hillside. They should last for years.

We had great crops, no erosion, and should be able to plant early this spring. My BIL couldn't believe how much I got out of those beds!