What should be my first Omega?

Tough call. Both are great watches in their own right but very different from each other. I have a Speedy and want to add an AT to my collection. I don't want to oversimplify my answer here but you don't have a blue dial in your collection. That could be a consideration. Good luck.

Speedy makes the most sense to me given the watches you listed. Adds a chronograph, new brand at same time, and it’s nothing like the rest of your watches.. To me, the AT is an alternative to the Explorer, and Seamaster is an alternative to the BB58.

I've always liked the design of the AT, especially the horizontal "teak" lined one you posted.

Globemaster

If you’re going for the standard collection, gotta add THE chrono; Speedy.

I have been lucky to be able to own all 3 of these Omega watches. I bought the Speedmaster first, then the Aqua Terra. The Seamaster never appealed to me at all until I actually saw it in the jewellers, that changed my mind.

I'm not one for details on movements, complications etc, I'm unashamedly all about the aesthetics of the watch. Does it look good & does it feel good on the wrist? For me, the Aqua Terra is the best looking of the 3 & it's also the most comfortable on the wrist.