On my livestream I regularly emphasize the importance of not reincarnating right away at 30 because there is a disproportionate amount of reaper xp available in legendary questing. I get about 1 million reaper xp per life, with about 90% of that being from legendary questing. I spend about a week at cap and do most legendary quests each life, skipping a handful that are long, grindy or otherwise a pain.
The reason that reaper xp is disproportionately high at end game is primarily 2 reasons. The level of the quest is part of the reaper xp formula, therefor higher level quests provide more reaper xp. Additionally, legendary quests (base 30 and above) have a x2 multiplier on reaper xp. Add first time bonus, ship buff, Master's Gift and the biggest xp pot you can and you have huge reaper xp.
For best results, you should do as high of skull as you can but even if you can only do low skull or even just 1 skull, it's still worth doing as many first time legendary quests as you can because the reaper xp is so big.
If you don't want to spend a lot of time at cap, I suggest making a list of 20-25 short quests that you can spend a day or 2 doing. This alone will drastically increase the amount of reaper xp you get over the course of a life and especially multiple lives. It is important to take advantage of that first time bonus that is only there once each life. Reaper xp per quest drops significantly when repeated.
If you prefer to do a lesser number of short quests, here is a recommend list of short, easy, fast quests to choose your favorites from:
Cogs:
- Wraithcallers
- Ruinous Schemes
- Thralls of the Fungus Lord
- Scavenger Hunt
- Magma Must Flow
- Security Detail
- Roll Call
Ravenloft:
- Into the Mists
- Death House
- Fresh Baked Dreams
- Final Vintage
- Wrath of the Earth
- Raven's Bane
Gatekeepers:
- Home of Memory
- Heart of the Problem
- Rosemary's Ballad
- Sacred Bounty
Feywild:
- Knight Who Cried Windmill
- Thornwright
- Legend of the Lost Locket
Saltmarsh:
- Back to Basics
- Down You Go
- Swim at Your Own Risk
Docks:
- Members Only
- Offshore Account
- Saftey in Numbers
Isle of Dread:
- Captive of the Hidden God
- The Curse and the Captive Crustacean
- Trouble with Tar Pits
- Wild Thing
- Trials of the Triceratops
Vecna Unleashed:
- What Dreams May Come
- Party 101
Other:
- Subversion
- Tavern Brawl
- Grim and Barett
- Lost at Sea
- An Element of Chaos
- Seizing the Dawn
- Assault on Summerfield
- Blockade Buster
- Catastrophe
Quests that I skip at end game: Toxic Treatment, Soul Survivor, Best Laid Plans, Slave Lords and most of TOEE. Sometimes I will just do them on lower skull.