Which secondary rider is the strongest?

Hm... Hyper Gatack maybe?

I'd say probably either Knight, Meteor, Baron, or Brave. I'm basing this on a combination of fighting skills and power up forms.

Chalice because undead.

In the shows, Showa connections are:
OOOs has the 1000th episode of Kamen Rider in which a Shocker Combatman is the main villain for 2 weeks. I believe some OOO's related plot stuff is in it to making it hard to remove.

In episode 2 of Fourze, he takes the name of Kamen Rider after seeing footage of Showa Riders. I'm sure Rider 1 was in those clips somewhere. If not, Riders he's had long time team ups with are.

As mentioned, Ghost has Rider 1 cameo in an episode leading into the Rider 1 movie.

Even if you consider the 00's to be alternate universes (which at the time was the easier assumption), the 10's are very well connected to each other and to Showa beyond the movies. It would be silly and needlessly complex to try to explain away these connections without just using the movies.

Who all subbed the Kamen Rider 4 specials? Did only TVN do them or did Over-time do it also?

I was considering connections to be "full plot is connected in some way." Ghost and the ZyuOhgers fighting a Shocker monster is not that.

Aside from the movie specials Shocker never really surfaced again as the mainstay bad guys in the actual Heisei era TV shows. Of course, Foundation X acted like they Shocker to a certain degree but they never said that they are the reincarnation of it, etc. At least for now.

hopefully foundation x will return in KR Build and having a proper conclusion instead of just suddenly disappear.. haha

Fastest rider?

Faiz Accel
Kabuto Clock Up (not hyper)
OOO Ratoratah
Wizard Infinity
Drive Formula