While little is known about Android 18's power, I feel she has a few feats that potentially puts her on par with Base Goku / Vegeta (ToP). Some of these feats even suggest that her power may actually sit a few tiers above the duo's base forms and is perhaps somewhere between base and Super Saiyan, particularly after defeating Giant Ribrianne.
Defeating Ribrianne - While Ribrianne's power can seem a bit inconsistent at times, I feel she's comparable to Base Goku / Vegeta at the lowest. Factor in her giant form, and she should be a good deal stronger than the Base Saiyans. Personally, I feel she is worlds ahead of anything the Buu saga had to offer, as far as strength is concerned; SSJ3 Gotenks couldn't even scratch Copy Base Vegeta, an incarnation of Vegeta that likely pales in comparison to the version of Base Goku that Ribrianne had to fight.
During her initial battle with Android 18, it seemed pretty apparent that while #18 held the edge in power, they weren't too far apart from each other. Upon realizing her giant form, Ribrianne was immediately able to gain the advantage. After struggling for a bit, Android 18 also appeared to have obtained what seemed like a power up. I mean, how else could she have defeated someone she was flat-out helpless against just moments before?
There's also her battle with some of the Pride Troopers, where she was able to casually toss Tupper, an opponent that was apparently crushing Base Goku. While one could argue that Tupper increased the amount of weight against Goku, this was never explicitly stated nor implied at all; therefore, the more logical explanation in my opinion is that Android 18 was simply strong enough to make quick work of Tupper.
So taking all of these points into consideration, I feel Android 18 probably started off at Base Goku-tier, but actually surpassed it via her power up. The following is what seems like a pretty decent interpretation of where she stands in my opinion.
SSJ Goku > Android 18 (vs Giant Ribrianne) > Giant Ribrianne >> Android 18 > / = Ribrianne = Base Goku / Base Vegeta >>>>>> SSJ Vegetto > Buuhan > Buutenks > Ultimate Gohan > SSJ3 Gotenks > / = Super Buu > Mr. Buu
Defeating Ribrianne - While Ribrianne's power can seem a bit inconsistent at times, I feel she's comparable to Base Goku / Vegeta at the lowest. Factor in her giant form, and she should be a good deal stronger than the Base Saiyans. Personally, I feel she is worlds ahead of anything the Buu saga had to offer, as far as strength is concerned; SSJ3 Gotenks couldn't even scratch Copy Base Vegeta, an incarnation of Vegeta that likely pales in comparison to the version of Base Goku that Ribrianne had to fight.
During her initial battle with Android 18, it seemed pretty apparent that while #18 held the edge in power, they weren't too far apart from each other. Upon realizing her giant form, Ribrianne was immediately able to gain the advantage. After struggling for a bit, Android 18 also appeared to have obtained what seemed like a power up. I mean, how else could she have defeated someone she was flat-out helpless against just moments before?
There's also her battle with some of the Pride Troopers, where she was able to casually toss Tupper, an opponent that was apparently crushing Base Goku. While one could argue that Tupper increased the amount of weight against Goku, this was never explicitly stated nor implied at all; therefore, the more logical explanation in my opinion is that Android 18 was simply strong enough to make quick work of Tupper.
So taking all of these points into consideration, I feel Android 18 probably started off at Base Goku-tier, but actually surpassed it via her power up. The following is what seems like a pretty decent interpretation of where she stands in my opinion.
SSJ Goku > Android 18 (vs Giant Ribrianne) > Giant Ribrianne >> Android 18 > / = Ribrianne = Base Goku / Base Vegeta >>>>>> SSJ Vegetto > Buuhan > Buutenks > Ultimate Gohan > SSJ3 Gotenks > / = Super Buu > Mr. Buu