I recommend hitting Utopia and the episodes with it before hitting the 12th Doctor.
Remember....evil Time Lord. (though to some the Doctor is the evil Time Lord)
^ Agreed. That's the three-parter that finishes Season 3. Other
suggested Ten episodes you might want to visit one day soon (esp. if rumors about the upcoming Season 9 prove true)...
The Fires of Pompeii - Ten and Donna visit the tragic Italian city on Volcano Day, and meet a couple of familiar faces (familiar to
us, but not to the Doctor - not yet, anyway...)
The Doctor's Daughter - yep, exactly what it says it is. And Ten, with Donna and Martha's help, has to deal with literally
instant parenthood. (The titular daughter is played by the real-life daughter of Doctor Five - Peter Davidson - and ends up marrying David Tennant IRL.)
The Stolen Earth/Journey's End - Ten and Donna team up with every one of the supporting characters of the series so far (including Rose!) to defend Earth against an all-out Dalek invasion. Here's where you'll meet a monster even worse than the Daleks - the monster who
created them. And Ten has his little 'vanity' issue that Eleven mentioned in his farewell episode...
The End of Time - Ten can't escape his fate. The evil Time Lord from the above-mentioned three-parter won't let him. Plus you'll meet the creator of the Time Lords, a man with the face of James Bond - a man equal parts George Washington and Joseph Stalin.