The tanks we've used have changed over the years as our diving has changed.
Currently:
4 LP 85s
8 LP 95s (for sale)
4 LP 45s (for sale)
6 Steel 72s
2 2 Liter Faber FX 15s
3 AL 80s
3 AL 13s
1 AL 6
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Starting out in OW in 1985, I had two steel 72s. I still have both of them today, along with another four I picked up along the way. They are still excellent tanks for single tank diving with better buoyancy traits than AL 80s. We also use them for side mount dives in OW and in smaller cave passages. They again have delightful buoyancy traits for side mount diving and when filled to 3000 psi offer an honest 86 cu ft capacity, 9 cu ft more than an AL 80.
We also own three Luxfer AL 80s (but they've been on loan the last couple years to an instructor who uses them for OW students). We used them as stage bottles for longer deco dives in caves, and they'll work just fine for side mount rebreather bottles as well for diluent/bailout gas.
We own 4 Cyl-Tech/Hy-Mark AL 72s and they are extremely nice to dive, with the same 7" external diameter as a Steel 72. They are only 1/4 smaller in diameter than an AL 80, but the difference is noticeable. They also make excellent stage bottles and diluent/bailout rebreather bottles.
The Steel 72s, AL 72s and AL 80s are all light enough to be floated with an SMS 50 (22 pound wing) in a 5mm wet suit, along with cave lights, reels, etc.
We own four Faber LP 85s that we use as side mount tanks for cave diving. They are once again 7" diameter tanks and help make small passages feel bigger. With a 3600 psi cave fill, they provide 115 cu ft of gas each, 230 cu ft per pair, only 30 cu ft less than a pair of cave filled LP 95s. They are heavy enough that the SMS 50 won't float them with a wet suit, however they are nearly ideal for side mount cave diving with a dry suit and they make excellent diluent/bailout bottles for our side mount rebreathers.
We have four pairs of LP 95s, and we used them for about 5 years as doubles for cave diving, and then for the last 5 years as side mount tanks for cave diving. They are however currently all for sale ($400 per pair, with bands and isolators available) as they are no longer the best fit for the dives we do.
We had four Worthington X7-100s that were absolutely superb tanks for NC wreck diving, holding 100 cu ft for about the same weight as an AL80, once you factored in the additional weight needed for AL 80s in a dry suit. We rarely do that kind of diving anymore, and if we do, we'll use our LP 85s, or our rebreathers, so I sold all four to the local dive shop about a month ago.
For smaller tanks, we have four LP 45s that we used for O2 decompression bottles, with an eye toward possibly using them for very small passage side mount diving, but I don't think that's a direction we're moving in, so they are up for sale ($300 each, O2 cleaned for the pair just back from hydro and $250 each for the set due for hydro next year).
We have a pair of AL 30s that have been our primary O2 decompression tanks for nitrox and trimix cave diving. AL40s would be slightly better, but not enough for us to ever bother switching.
I have a Luxfer AL 6 and a Luxfer AL 13 that I've used over the years as an inflator bottle for cold water trimix dives. I'm still using the AL 6 as a potential redundant source of diluent for my side mount rebreather, and the AL 13 is currently doing duty holding pure helium for our Air Swimmers.
We also have a pair or Worthington AL 13 tanks that serve as O2 bottles for our rebreathers, along with a fair of Faber FX 15 15 cubic foot (2 liter) steel tanks that have the same job. The difference in buoyancy when full is only 0.3 pounds so the two types are essentially interchangeable. If I did it over again, I'd skip the twice as expensive Fabers and buy twice as many AL 13s.
