My current line up of coil choices in order of preference...
For land- 12x10, Ultimate, 10" Tornado, S-12. Haven't used S-12 yet though but it seems very popular among water hunters and I suspect part of that is it isn't too much drag. I bet it would jump over my 10" Tornado as to which I'd prefer for land use if I had one and maybe even surpass the Ultimate as my go to coil on land, but from what few guys I've talked to who owned both it and the 12x10, I suspect I'd still prefer the 12x10 for my #1 land coil. Some feel the exact reverse of that though, as it's an outstanding coil by any measure from what most say.
For Water- S-12 (I Suspect), 10" Tornado, 8" Tornado (SEFs are too much drag for me in the water)
For Super Trashy Land Sites- Tornado 8" (7 & 1/4"). Haven't used this one yet either but I hear nothing but praise for it's separation in super bad trash and that it's a very deep coil for it's size.
For Huge Fields Or Beaches Where Coverage Is Primary- While I enjoy the improved coverage of the 12x10 over the 10" Tornado, if I was a huge open field or expansive beach primary hunter, coil choice for that would be (once again in order of preference, & lightest to heaviest too)- 13" Ultimate, 15x12, WOT. Never owned a WOT but I'm basing my order of preference for me on weight and what I've read of some like Kered say about it compared to their 15x12, though there are some who like the WOT better than the 15x12, so both have their fans. It's a great coil with a well earned reputation, so I'm not knocking it. If you saw the one recent youtube video a guy banged a shotgun casing at I think 14 or more inches with it in somewhat bad ground with no problem. His friend has scanned around the tree first with his machine and saw no targets. Pretty impressive stuff.
Having owned a 15x12, it's for sure more stable more often than the Ultimate at higher sensitivity levels, but it's a good bit heavier than the Ultimate or 12x10, though still lighter than the 10" Tornado if you ditch the coil cover. It would be a tough choice between it and the Ultimate, but I really like the Ultimate too and it's weight and balanced feel for such a large coil just amazes me. It just "feels" good on the end of the detector shaft. If you are primarily a beach hunter though I'd probably lean towards the 15x12, as a few are saying the Ultimate is not very stable on their salt water beaches. On my mineralized beaches it's well behaved, but I do have to turn down the sensitivity lower than the 12x10 to keep it from nulling out on the ground matrix as much. Think it's an awesome coil though in many respects. If I lost my 12x10 I'd for sure want an Ultimate, and in fact I intend to get one to sit along side my 12x10 as my loaner Ultimate goes back to the owner soon.