Seeds, although I got some plug plants of perpetual spinach and Pak Choi as they tend to get obliterated by slugs so I wanted to give them a fighting chance!
Raised beds warm up faster than ground beds in spring, so you can start seeds off a week or so earlier. I usually germinate seeds indoors on window ledges and plant them out when they're at least an inch tall with a few "proper" leaves.
You can't do that with root veg (carrots, parsnips, beetroot etc) as disturbing the root makes them grow wonky. Those seeds need sowing direct.
Peas grow easily with sticks to support the stems, lettuce too (if you don't plant too many at once and get a glut).
Onions are best grown from sets (tiny onions) rather than seed, but spring onions from seed are a good quick win.
Potatoes and sweetcorn are a faff for the amount of space vs crop yield.
Sprouting Broccoli takes up space for almost a full year, but will then crop for months if you keep picking it.