What do you recommend?

Many good sealants to chose from. Stick with a polyurethane, tri-polymer, or poly-ether. I have used many different ones over the years my current favorite is Bostik 1100. Others I would recommend are Sikaflex 505, Trempro 635, and Chemlink M1.

I've used Proflex with great results. Be careful prepping seams your going to caulk. Acetone, for instance, will take the paint off some trim. It also takes a very small bead of caulk, maybe an 1/8" at the most.

I tried some 3M 5200 marine caulk and it seems to work very well. They make a quick dry 5200 also. The regular 5200 takes a long time to set up. The product is for the marine environment and stays flexible. Also a recommendation about using masking tape along the joints. This keeps the caulk confined to a narrow area and an neater job. After you get done you just pull off the tape. I've seen some folks caulk and use their finger to smooth it out. This leaves a very wide unprofessional looking job. Use masking tape!

I used Sikaflex 505 UV caulking a few years ago and it still looks like new. It's available at camping world, or amazon.com. It has a UV protectant and is very durable and mold resistant. Kinda expensive but will outlast any other caulking on the market.

Dicor NON-self leveling

Proflex

I prefer Proflex but can be harder to work with then Dicor