This depends on many factors. I assume you want to grow forage for ruminants like cows or sheep.
You want plants that will grow a good yield of nice leafy biomass that stays leafy long enough for your animals to enjoy. You also want species and cultivars that will grow well in your climate and soil conditions.
A very important question to ask is do you want seeds that will:
A) grow perennial plants that are the final succession in your pasture, or
B) seeds that will help progress succession to what you want, and grow lots of tasty forage in the mean time.
I find people tend to pick A, but they do not take intake account the state their pasture is in and whether the perennials would do well. If the carbon-to-nitrogen ratio is very low aka very bacterial dominated soil, or low organic matter, compaction, low or high PH, any of these will cause typical perennial forages to struggle.
The best way to tell is by what's already growing there. If there are lots of weeds especially annuals like thistle, ragweed, pigweed, then it is likely an indication of soil conditions that are more optimal for weed-like annual forages than perennials. And these weeds are not necessarily bad! Cattle and sheep especially can gain a taste for them with enough exposure, and they can be nutritious in the right stage of growth. But it is important to see what is already growing as these will be an indicator of the soil conditions. A soil test will help to confirm.
If there are less than optimal conditions for perennial forages, I recommend growing robust annual forages that will produce a good yield of palatable biomass while moving soil conditions to what you want. You can even throw in some less palatable species that have benefits for the soil, just don't add much to avoid them taking over the pasture. To choose which seeds I highly recommend the SmartMix tool that can be found here:
https://smartmix.greencoverseed.com/. It allows you to choose both your goals and growing conditions to choose a suitable seed mix.
Whether you grow weeds or introduce annuals simply letting them grow will do a lot to improve soil conditions and advance plant succession. After just a couple years of doing this you may even see conditions change enough that seeds in the latent soil seed bank will emerge and you may be very surprised by what comes up. Once you see this then it is a good time to consider which perennials you may want to seed.