Every girl in the entire blogging universe must have heard of Benefit's Porefessional Primer (if not, I think you need to get yourself out from under that rock pretty sharpishly!) I adore this product - I can really see why it gets so much praise and why it's the UK's best selling primer. I've been using it daily and really have fallen head over heels in love with the stuff.  It really does do a smashing job of keeping makeup in place as well as creating an amazingly smooth, flawless base. I find it works particularly well underneath my eyes and around the sides of my nose - usually places I find I have to top up occasionally but wear-time of my foundation/concealer is impressively increased whenever I use this. The oil-free, balmy formula feels so silky and soft to apply - it smooths across the skin with ease and feels so lightweight - it instantly makes your skin feel velevety soft and smooth to the touch, as well as reducing the size of pores and creating an even skintone ready to apply your base.
I don't fall in love with primers easily so it really is saying something that I enjoy using this one.  It even smells lovely. As well as prepping skin for makeup, it works well at minimising shine and reducing any redness or open pores - meaning that it can be worn alone if you're having one of those 'bare skin days'.  In my opinion, a primer's main purpose should be to keep your foundation in place for longer during the day, and this has been fabulous at doing just that - if you're struggling to get your base to stay put - I'd really recommend trying this! Would I purchase this once it runs out? Yes indeedly - I would. A little goes a very long way so your investment in the product is fully justified - you only need a tiny blob to cover your entire t-zone. It does what it says on the tin and keeps my makeup looking flawless for longer, and for that, it gets a massive thumbs up from me.  
What's your fave primer?