First of all, the Culture books are about a post-scarcity society capable of doing just about anything we can conceive of. A plasma weapon to them could be the height of retro-chic, an affectation like Gibson’s Johnny Mnemonic hand-making a shotgun to go crude and surprise his enemies because he’s a “very technical boy”.
With realistic tech, I think we’re still going to stick with kinetic weapons for a long time. We might finally switch to caseless ammo sometime in the next 20 years. Lighter, but faster projectiles will probably also come into use around the same time. The advantages are similar to switching from 7.62 to 5.66 for most weapons; smaller, lighter ammo means more rounds for the same loadout weight.
Smart rounds are quite a ways away. I wouldn’t bet on having any self-guided bullets for at least 50 years, if ever.
Railguns are interesting in concept, but require a game-changing breakthrough in energy storage or generation, one which hasn’t happened yet, and may not happen in the foreseeable future. Emplacement weapons, probably. Handheld, not in our lifetimes.
Lasers, see above, plus add problems with wavelength and attenuation. Oh, and a hit anywhere on a human body with a laser capable of burning a hole in something with only a fraction of a second of contact will probably just blow out a big goddamn chunk as all the water in the tissues explodes into steam. You’d get either “does nothing because the armor protected me” or “my arm and half my torso just got blown the fuck off”, with pretty much no in-between.
Plasma, well, if you could do that, then you’ve probably got the fusion problem worked out, which means that lasers and railguns are old hat already. The only realistic power source I can think of for a plasma weapon is a nuclear or anti-matter reaction, and if you can miniaturize something like that enough to carry around, you are probably way beyond blowing holes in people with personal weapons.
Unless you’re trying to make a statement, or having “fun”. And so we come full-circle.