125 because some builds just can't develop properly at lower levels. For example, quality builds are at a disadvantage at SL100. Also, there are some odd builds like Velkas or the Divine Crusader Nuts came up with that really can't properly shine at 100.
Also, at SL 100 it is extremely easy to make builds designed to OHKO people. A bit too easy, at 125 you've got to put a bit more effort into it.
My fully fleshed out darkwraiths generally cap at 115 to invade most people upwards.
My fully fleshed out darkmoons generally cap at 130 to invade most people downwards.
And I've made too many level 4x, 5x, or 6x darkwraith characters to count (invading in Tomb of the Giants is too fun). --- Current Dark Souls Build: Demon's Souls Fat Official Platinum Trophy Victory Lap PSN: wdh316-=-=-=-=-=-Join the Cult of Cody's Hat Today!
I'm honestly a fan of 90 for the same reasons listed for 100. 125 is too much of a pain with all the vit gouging. I could honestly probably scrape a few more levels off and probably get down to 80. PvP is much more satisfying at those lower levels.