What's with "the best". No such thing. Best is what works for you best. So terribly subjective.
I know Coldplay quite well and the type of sounds you are refering to in their mixes come from popular software synths/ libraries like Spectrasonics Atmosphere and NI Absynth. I think I even heard a few familiar presets here and there while listenning to Coldplay.
If taking a laptop to gigs is a possibility for you then there you have it. Two of the finest of the ambient/ atmospheric synths. Sure there's more out there but I'd rather speak from experience.
Hardware synths can produce sounds of this type just as easily. In fact my virus TI is capable of some of the darkest and moodiest in ambient. Quite capable of atmospheric airy sort of stuff as well. Same can be said for the rest of my rig. Fair to say that almost any synth with flexible enough tone architecture should suffice.
My advice to you is this. Go to your local music store and tell them what you're after. Try everything you can. Only then you will have some idea of what's out there. Talking about synths in forums is great and we love to do it here but you'll have to do some homework too. No opinions in forums beat real hands-on experience.
cheers,
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A gentleman is one who never hurts anyone's feelings unintentionally.
- - - Oscar Wilde