I would defo recommend Amsterdam as a base. Schipol airport is only 15 mins by train and it is one of the most central airports that flys everywhere after Heathrow. It’s close to Scandinavia, Belgium, Germany and France, so you could catch the train a lot, plus it’s only 25 mins to UK/IRE and an hr from Spain and Italy. And lastly, it’s an absolutely brilliant and fun city.
On iPad using BigFooty.com mobile app
Side note: Amsterdam is fun but it gets old real quick. I was there in "low" season this year and it was still bursting with tourists - could hardly move. Yeah the cafes are cool and check out the red light district for a night to see how ingrained prostitution is into their lifestyle - but you only need a night to do that. But it's just so over run now and pushing thru the crowds - mostly loud drunk English lads - ruins the experience somewhat. I've visited Amsterdam three times now but the last two times only as a base to go out and see Haarlem, The Hague, Utrecht, Zaanse Schans and Rotterdam.