TL;DR Whatever tier you can run under 3min and at least 10 levels above the glyph you want to level if any.
The XP reward in pits scales slower than monster HP with increasing tiers. For efficient leveling you need to optimize time/XP gains. It might be better to run pit 60 over 100 even if you can beat both. In my testing it's usually best to run pits you can finish just under 3 minutes.
If you are also looking to upgrade your glyphs, make sure to run tiers that are at least 10 levels higher than the glyph you're looking to level. That guarantees a 100% chance of glyph level up.
Glyph leveling is no longer capped at pit tier 100 since patch 2.0.5.
Consult the D4Pits tool to optimize your XP and Glyph leveling.
I'm using values from Torment 4 with all seasonal bonuses, +8% monster XP elixir and +15% monster XP opal. Monster XP makes a surprisingly small difference as most of the XP from Pits comes from the completion bonus (included in boss XP on D4Pits).
Boss XP seems to be super consistent across runs. Monster XP varies, but it tends to average around 60% of boss/completion XP. A little more in lower tiers, a little less in higher tiers.
Contact me on reddit (u/korpsik) or send me a message on bnet (pajus#2460).
XP data for pit tiers above 120 would be welcome.