A good primer should help your foundation go on evenly, and last all day without caking, streaking or wearing off in patches.
Here's a round-up of my favourites from the last six months.
This Works In Transit Camera Close-Up (£30.00)
This definitely has my number one spot. It isn't strictly a primer: it's a mask, moisturiser and primer all in one, and does every job perfectly. I love This Works; they have so many beautiful things (you must try their deep sleep range, if you haven't already) and this is one product that I buy regularly and have been doing for a long time.
With argan oil and other plant extracts, it's a beautiful natural product which ticks all the boxes. As the name suggests: this works.
Smashbox Photo Finish Primer Oil (£28.00)
I don't think any list of good primers is complete without a Smashbox mention. Possibly not good if you have naturally oily skin, but with combination skin this works a treat.
I think you get a lot for your money, and it lasted me ages. It made my skin exceptionally soft, and my foundation stayed put all day long. Definitely worth the splurge.
MAC Prep + Prime Skin (£21.00)
Surprisingly, since MAC are known for their price tags, this is the cheapest of the lot.
I wasn't blown away by this at first use, unlike the two above, but after about a week the difference in my skin was very noticeable. I goes on really smoothly, and lasted a long time since you only need a tiny amount. I found this one gave the best results for my skin overall, and the amount of dryness was visibly reduced after a couple of weeks. I use MAC's Fix+ spray to set my make-up, and the two work ideally together.