I do think Alonso is the best there is at the moment, but there's a very real danger that in the future that won't be the case. He only has two World Championships- some could argue he should have won more (although I don't think he had the best car in any of his near misses). That could also create a problem, has he made the right career choices? Moving to McLaren sounded good at the time, but turned into a disaster which forced him back to a pale, shadowy Renault. Ferrari looks like the right fit for Alonso's personality- but they aren't producing the car to win at the moment- and I'm sure Alonso feels that that is the most important thing.
His career could go the way of Senna's- we all knew he was excellent, but his car couldn't win the championship in the later years whilst others walked off with championships far too easily (Button, Vettel compared to Mansell, Prost? Might be a stretch...).