Tier 5 - Tier 5 really depends on map, T1 Heavy/KV-1/KV-1S could all be used on city maps, while the T67, T34 can be used on maps where DPM is needed, and more obscure tanks like the Ram II, Leopard (With the autoloader) can be useful.
Tier 6 - T37, T37, T37. Hellcat is basically useless, just a free kill for the other team. M6 is a good heavy on maps where a heavy can be used. The cromwell is basically the only good medium at tier 6 for tourneys. There are some other tier 6 that could be used if you were required to bring a all heavy tank lineup, but that doesn't really leave you many options on which heavies not to pick anyways.
Tier 7 - T29 is the main heavy tank that is used, but the IS-2, IS, Tiger and KV-3 could also be used in its place if gun depression isn't an issue. For mediums T20 and Comet are the best as they are really the only two decent tier 7 mediums. For light tanks Bulldog is the best, but if someone doesn't have one and you are more casual, a T71 could be used in its place.
Tier 8 - Tanks that will almost always be needed is Ru 251, 13 90/50 100 and IS-3. Then you get into tanks that are more map dependent like the Pershing, T32, 416 and 110. The roomba and T69 I feel are not used very well when brought by teams, as even most Gold League teams don't bother with them anymore, and if they do they've got a strat they're very confident and are being used by players who can make sure they're used correctly.
Tier 9 - E75/E50 seem like they will always be the main Heavy/Med tank combo, but there are a lot more tanks that could be used better a lot more in certain situations.
Source: 10k gold from almost every skirmish in past forever