Truphone out of the UK is ideal for mobile nomads. You get numbers in up to eight countries, and in 66 countries it’s just like being in any of the eight. Cost is a bit higher but you get to keep your local numbers and when your local, you’re also hitting local pops so throughput is the same as if you were using a local SIM card.
Gigsky offers a global sim for data only.
Another great option is to have a dual SIM card phone like the OnePlus2 or 3 that just came out. Keep your local number from home set up to deal with those calls for authorization of charges and to have your friends or family back home reach you on…and use a Local SIM country by country.
As I live on iPhones buy the world edition, not simply an unlocked one in your country of choice. The world edition has all the frequencies for LTE.
Another tip is to look at the deployment of new network technology. Many countries are still 4G in USA terms vs. LTE, meaning its HSDPA or HSDPA+, which are older technologies. For example, I was in Amsterdam and conventional wisdom was to go with T-Mobile, but really Vodafone had the faster network. In Prague it was the same way. In Portugal, it used to be PT was the best for Voice in the Lisbon area while Optimum (they have both changed names) was far better on data (LTE). But Vodafone invested millions in upgrading their network and was last year the better choice, and the network wasn’t as crowded, and offered better coverage in Lisbon proper.
Over in the UK, Three is the best because beyond their own towers/masts/antennaes they have half of the capacity of Orange which is now merged with T-Mobile to be EE, so in essence 3 has the most complete coverage in the UK and the fastest in the ground network.
In Spain I love Yoigo because they are the most cost effective, but Orange offers better speeds. In France Bouygues is best, but I do enjoy the ease and efficiency of Orange, where it used to be SFR which has gone to crap on iPhones and has a problem with Gmail for some reason.
In Austria last year there wasn’t any prepaid LTE so I went with T-Mobile voice and data on one phone, A+ on the other and my iPad. I had coverage “most of the time” out in Wine country.
Hope this helps.