In the latest Vanishing Point series, the Time Masters are afraid of RIp, because of all the tech he has in his lab. Tech a million years from now. Anyway, this should be a good prep battle.
Doom has become a competent time traveler over the years, but he's got a ways to go if he wants to catch up with Rip imo. Where Doom is hesitant to do anything that could potentially affect the timestream, Rip casually (and purposefully) involves himself in cross-temporal-spanning events on a regular basis. He also seems to have a hobby of collecting advanced weaponry on some of his journeys. A good example of the uber tech Rip has in his arsenal is "Suspendium" -- a material so strong that it was able to contain Mr. Mind when he was a multiversal threat.
Tbh, I think it's only a matter of time until Rip is revealed as an essential part of DC cosmology. The dude already has an understanding of time/reality (on all levels) that makes even the Linear Men look stupid.