ISU to me is easily the best T8 TD and the easiest to play. Even more so into higher tiers. Sure it doesnt have bouncy armor, but then which does? The T28 has decent armor but its extremly slow. T28 prototype has resonable armor but the turret isnt that strong. Mle 48 on paper has decent armor, but everyone just aims for the weakspot and pens without fail. (This tank also had its penetration dropped in 8.6). Ferdy has ok ish armor, but with premium ammo around its very easy to pen it, as most t7/ t8 tanks have more than 200mm of penetration with premium ammo and can just penetrate the superstructure. I dont have any exprience of the SU101 or the british one (except it has a nice easy to pen weakspot on top).
SU-101 is one of the worst of the tier 8 tds, it has the worst gun depression possible, but other than that its not so bad all round, but doesnt work, a tank should excel in at least 1 area, DPM or alpha, and be able to use it, the lack of gun depression means your positions are reduced drastically. It is however pretty fast so you can rush certain positions but it can go terribly wrong.
The AT-15 is a trap tank, the reload time is 4.21 seconds (without BIA), if a tank decides to pop out to shoot you, you can track him and keep him tracked and keep shooting until he is dead. the armor is good as long as they dont aim for weak spots.. But ideally with this tank you must attack hard. This tank is not one you want to face frontally with any other tier 8 assuming both are full HP. Many times people have asked me if this tank was an autoloader.
The gun traverse is amazing also, you can almost shoot around corners.
The other good thing is people dont realise the reload is so fast and the gun traverse is so good that when you shoot someone and another guy takes this chance to pop out to take his shot on you, you are already reloaded and aimed.
These are my stats for the tanks discussed.
ISU-152= 2890 AVG DMG / 1.92 kills / 1130XP AVG
AT-15 = 2391 AVG DMG / 1.95 kills / 1174XP AVG
SU-101 = 1591 AVG DMG / 1.41 kills / 987XP AVG
(note the XP on SU-101 was pre-8.6 so i buffed it 25% to compensate as the previous 2 tanks i grinded post 8.6 Original =790)
I have not played the AC, i will do soon, but IMO the armor is the best of the tier 8 TD's, and the weakspot is not easy to hit over distance if you wiggle a bit.