Now that the move is official, I think it's a great question to debate.
If we're talking about Henry at the peak of his career, I think there's no doubt that he's the best. Even when you consider the all-time greatest players ever to step foot in MLS at one point in their career, I don't think any come close to Henry's stature. While I'd love to make an argument in favor of Tim Howard, Marco Etcheverry, Eddie Pope, or even Ryan Nelsen, none has at any point in his career been the runner-up FIFA Player of the Year, or been the Football Writers' Association Footballer of the Year on three separate occassions. Dude's really got a nice resume.
But what about right now? Henry is coming off probably the worst season of his career, being relegated to the bench at Barcelona and doing little in South Africa for his pitiful national team. At 32, he is not old by any means, but he has unquestionably lost a step from his peak at Arsenal.
I don't expect him to compete with Landon Donovan for the MVP award this season, but will he be the best ever to have played here? Hopefully he'll be in MLS long enough for this to truly be a good debate.