I agree, I think out of the top three here it's the best one, but I feel that Tolle's would generate a better discussion. Having said that, and looking at the results so far, I've changed my vote to Frankl's just to avoid a discussion on Chimp Paradox.
I'm quite excited about this to be honest - you'll see me quite active on the forum while this is going on.
The thing is, part of the "present moment" shtick that Tolle focuses on works only if you have other larger framework elements in place. Frankl says in
The Doctor & The Soul:
"For healthy living is living with an eye to the future. In 'practice' it was often not to difficult to encourage some prisoners by turning their attention towards the future. For example, a conversation with two camp inmates disclosed that both of them were haunted by the feeling that they 'no longer had anything to expect from life'".
Without this directionality towards the future which manifests itself through goals, you lose motivation, and you end up sitting somewhat passively in the present moment, just like Tolle did, on a park bench for ages. You may feel GREAT inside, but this is nothing more than pure hedonism - just like sitting drugged the whole day in your house. Don't get me wrong, focusing on the present moment when engaged in some task, and not daydreaming or aimlessly thinking about the past is a good thing. But it's not enough for peak performance or psychological well-being.
Tolle also glosses over some of the metaphysical aspects of time. For example, it is true that the future & the past exist only in the present moment, but they do exist nevertheless. There is some element of necessity that the past brings about - you cannot retroactively change your past. And that necessity is very real in this present moment, which subsumes under it both past & future.
So while you may, for example by some accident, forget your entire past, you cannot escape its necessity - it still remains your past, whether you psychologically remember it or not, and its consequences remain with you in this present moment. Take the example of a slave from say Roman times. If he gets in an accident and he loses part of his memory, he may no longer remember his past as a slave, but his present is still restricted by the necessity of his past.
I personally find that the existentialists (and I count Frankl amongst them), really did get the core of the human condition right. Man is rooted in his past, but may nevertheless authentically project a different future for himself through his creativity & ingenuity. And in some sense, the future itself is more real than the present, because it is that which directs the present moment, and gives it something to aim at. This orientation towards the future, or what someone like Heidegger would call being-towards-death, is part of what makes the present moment invaluable, and different than the present of a vegetable.