No spoilers for the latest leavetaking,please.Some of us still haven't seen it.
There's two ways to answer this;the regeneration itself,or the story that surrounds it.For instance,Planet Of The Spiders is pretty dull,but the changeover scene itself is well handled.
Poor Colin Baker didn't even get a curtain call;it was done by Sylvester McCoy in a wig (hence he "Trial/Time"entry).At least McCoy got a regeneration in the TV Movie.Even the War Doctor got his at the end of Day Of The Doctor.,turning into the Ninth Doctor--and the Eight Doctor finally got his in Night Of the Doctor.Everyone but the Sixth.Poor guy.

Tennant

Davison.

Because, cleavage.

Eccleston's is my favorite. His speech was both expository and poignant at the same time without going overly long and David Tennant was so clearly what the Eleventh Doctor would be in also just his few words. And it wasn't over the top, like his exit.

Second place is probably McGann to Hurt.

This is a hard one for me I like most of em. I was edging towards Davison because that was basically the best Davison episode of the Davison era IMO despite havin Peri in it...but she hadn't had time to get completely annoying yet so I liked her more at that point anyway.

I've ultimately settled on Pertwee because I really did love that end scene and even liked parts of that episode...though yeah it did drag. I liked the Eccleson one quite a bit too. This latest one had some very nice moments and I quite enjoyed what I saw of it. I would consider it a success somewhat, or at least not a failure in the slightest...but I don't think I'd ever vote for it as a best of anything. I don't really see that as a problem they can't all be the best and it certainly wasn't the worst (That might go to McCoy just cause it occured in that terrible terrible movie ) I can't think of many that I didn't quite like anyway.

The Caves of Androzani. Season 21 wasn't the best, but Davison went out on a high, and it captured everything a good Doctor Who story should have.

Agreed.
And Colin proclaiming "I am the Doctor whether you like it or not" (if memory serves)
was a favorite of mine in that it showed the complete converseness (word?) of incarnations.

Favorite regeneration scenes: Pertwee and Eccleston. Favorite regeneration episode: I'd say "The Parting of the Ways," but seeing "Logopolis" as a kid is what really clinched me as a Doctor Who fan. It was so weird and unsettling and portentous and sci-fi-ish. Recursive loops of TARDISes inside TARDISes!

He said it, but it was at the end of his first full story, The Twin Dilemma.

I really need to re-watch the early ones. My memory is just not serving.

That said, Eccleston's regeneration just tore my heart out. Not over long or maudlin, just rueful. A glint in the eye, a toss of the head and then a page flipped and he was Tennant, tearing it up! Of course then we were off to the overly long The Doctor Is Now Tennant sequence with revival by tea and a lost arm.

Colin Baker had the worst send off.......

I had to vote for Night of the Doctor ( i had waited 17 years for this)

In 7 minutes Mcgan gave a performance that showed what could have been.

The regeneration scene was heartfelt and effective

I've always thought that if they had been given time and the right material, Colin Baker and Paul McGann could have been two of the best Doctors. Instead we got Sylvester McCoy who I think was the worst Doctor, but was given some decent stories.

The lines after he regenerates are (nerd alert!):

Peri: Doctor?
The Sixth Doctor: You were expecting someone else?
Peri: I... I... I...
The Sixth Doctor: That's three I's in one breath, makes you sound a rather egotistical young lady.
Peri: What's happened?
The Sixth Doctor: Change my dear. And it seems not a moment too soon.

Yeah, Davison takes it for best departure adventure and Colin has the best opening of any Doctor. I mean, Harper directing a Holmes script, it simply doesn't get any better. Also, it was just the best Davison adventure he had, too bad it was his last.

Second place would be a tie going to McGann in Night of the Doctor and Hurt in Day of the Doctor, amazing stuff and both scenes unexpected.

Third place goes to Troughton in the War Games--big, fun adventure which first told us all about the Time Lords for the first time ever.

Time of the Doctor was a mid level adventure saved as usual by Smith's excellent performance.
Planet of the Spiders was a good romp and the most visually sedate regen ever.
Parting of the Ways was a decent exit for Eccleston.

On the lower end,
End of Time was ok but I never liked that incarnation of the Master and Tennant's incessant whining got on my nerves.
Logopolis wasn't very good or even interesting or even that well executed. It just ranks up there because Tom was leaving after 7 years and that was big news.
Tenth Planet was interesting for the intro of the Cybermen and the very first regen but aside from that, not that amazing an adventure.
Movie---eh.
Time and the Rani, worst of all. Colin was cheated.