Pick a big one with a good reputation and one that has been around for a long time. Your data is passing through their servers and data security should always be a prime concern.
I have strongvpn on one router and it has not missed a beat in years.
I use their cheapest package (~$55/year the last time I looked).
That's the same package I use at StrongVpn, just renewed it last month for another year. Hasn't ever missed a beat for me either. The one time I had a question, their online chat immediately connected and someone had an answer for me immediately that fixed my problem.