Obi-Wan Kenobi vs The Grand Inquisitor

Both at their peak and best :

1. Light saber duel only
2. Force powers only
3. H2H combat only
4. Combo of the all the above in a duel

Who wins?

Obi Wan stomps

Obi-Wan destroys. The Grand Inquisitor isn't suited for going up against Council-level Jedi Masters (save Jocasta Nu, who is just a middling Jedi). The fight might take a little while, due to Obi-Wan's defensive nature, but Kenobi would never be in any actual danger of losing to the Grand Inquisitor.

Kenobi stomps GI infinitely easier (and faster) than Kanan did.