@Monk, my suggestion would be to first decide your price point and if you want new or presmoked. If you choose presmoked, then you decide if you want really old like 1970-1980s vs more modern (older "tend' to have tighter draw, and those after 1990s have that open draw so often talked about. Then decide finish. Remember, ALL Castello pipes are made from the same aged briar, so they all smoke the same. If you want an inexpensive introduction Castello, sea rock 1990-2017 would be a good choice. If you want to go all out on your first Castello, then a new Collection grade smooth pipe would do you well.
Also, think about your own preference in stems. Castello stems are Lucite, and some can be huge with big thick buttons, and will drive you crazy if you are used to Vulcanite stems. Also, Castello's "tend" to be heavy, not always, but they like big pipes and if you are a clencher that will be something to take note of.
Opinions vary, but I can vouch for the following shapes, they are dandy and will be a good first Castello because of the weight, stem, etc. Again, just my opinion based on experience.
The pot with small stem (wonderful work horse if you prefer flakes)
https://www.smokingpipes.com/pipes/estate/italy/moreinfo.cfm?product_id=251904
I love this one for a first Castello. I have a few myself and reach for them often.
https://www.smokingpipes.com/pipes/new/castello/moreinfo.cfm?product_id=272902
The bent apple (that was my first Castello shape and I smoke it all the time, wonderful if you prefer ribbon cut or shag or English)
https://www.smokingpipes.com/pipes/new/castello/moreinfo.cfm?product_id=272651
The TINY billiards are awesome, not that common, but they are light and the stem can be clenched easily
https://www.smokingpipes.com/pipes/new/castello/moreinfo.cfm?product_id=272513
And the classic Castello is the shape 55 in Sea Rock. You can't go wrong with that (or any other shape you might be drawn to).
It's a RABBIT HOLE my friend. Have fun. So many options.