Relentless is best used on champs with really good abilities that you want to run as often as possible, but who also don't need to necessarily focus on a big nuke or anything like that. A very common candidate is Scyl, since every turn she gets, you get a free heal, a free buff, and you reduce the cooldown on your revive and stun. She's by no means the only option, but she's probably the most common application.

Regeneration on the other hand is much more specific. Realistically the only places I see it commonly used are in DT, especially DT hard, for some of the more annoying bosses like Bommal and Scarab. You *could* run it for an arena go-second team, but I usually find there's more value in other sets like Frost or Stun.

Relentless is also good for champions who already have skills or passives giving them extra turns, as on every extra turn they will get chance again to proc another extra turn.