The only oneworld airline with a comprehensive intra-European network and significant presence in PAR that will take you to all the destinations you mentioned is BA, so if you decide to go with oneworld you'll likely end up on their flights to LON pretty often.
Checking statusmatcher.com for leveraging your current A3 status it appears that airberlin may either match your A3 status outright or (more likely) grant you their gold (OW Sapphire) status combined with a challenge. Topbonus is a decent program, but beware of low earn rates for the cheapest fare classes on many OW partners, including BA. (find the chart here).

Even when flying cheapest economy fares, your stated amount of travel should give you enough tier points to achieve silver status in BAEC. Unfortunately, since you appear to not travel to North America, AA is not an option for you as they now require you to fly four aa-coded segments per year. And all other programs except Iberia's have low earn rates on the most discount economy BA fare classes, which you most probably end up booking.
So I'd go with BA. The tier points used to measure your status qualification is a bit more complicated than the mileage-based systems in place at most other airlines. I'd suggest reading the BA forum and browsing through the sticky introduction thread for clarity....

To accelerate achieving status, BAEC has one thing going for them: Even with cheap cash upgrades done online or at checkin (something I totally would do on any long-haul flight that has their new WT+ premium economy cabin .Well worth the small-in-comparison fee), you are credited tier points for cabin class flown - which will get you to silver status roughly two-and-a-half times faster than it would take with discount economy.
Example: A return flight from Paris to Singapore via London will net you 100 tier points in discount economy. You'd need six flights to achieve silver status at 600 TP. Upgrading the longhaul segment to Premium economy for about £350 one way will net you 220 tier points - you'll have silver status after just three such flights.