The original question has no answer; there's no "best airline" for anything. What route? Which class? Who do you have status with and could count on some sort of upgrade?
That being said, here's my take on the matter, from long experience:
Regular coach, domestic: JetBlue. That extra inch really does make a difference.
If you're stuck in a regular coach seat, no matter the carrier, there's a simple way of being infinitely more comfortable: take your shoes off. Losing an inch or so of height at your knees just adds that little bit of space you need not to have them crushed. Loosen your laces at the gate, and please oh please wear socks and take a shower beforehand
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Extra legroom coach: Really depends on the aircraft and configuration more than airline. I typically aim for an exit row over a regular premium economy, as that tends to have the most knee room, plus I can put my legs underneath the seat in front of me. If the aircraft has two exit rows one after the other, I take the rear one since typically the forward one doesn't really recline, so I know my knees are safe.
If the aircraft has a curtain or half a bulkhead in front of the premium economy, I may opt for that row instead. But if there's a full bulkhead, I'd rather be able to stretch out in an exit row than have some more supposed space in front of me.
Business/First: No winner here, as even very similar seats between airlines or aircraft could be vastly different for tall people. For example, CX J on the 77W is nearly identical to the seat on the DL A333; if anything, the CX is considered superior by most. For me though, the DL product was incredibly comfortable, while the CX seat literally hurt. All you need is a tiny little thing protruding somewhere, or half an inch off a footwell, and the seat is relative crud. Regular people may never notice these subtleties, but for someone who takes up every available inch of space, these things matter tremendously.
I flew CM J four times in one week, all in the same seat and the same aircraft type, but they may have all been different products entirely. Two flights were quite comfortable, while the other two felt like coach, legroom-wise.
In short: being very tall has a great many benefits and I wouldn't choose to be shorter were I given the choice now. But when it comes to traveling, it kinda sucks. No way around that.