Best lines to research first and why (just imo)

Hello, this is just my opinion which most will probably disagree with and goes by a different logic than many. Its not for anyone that cares about playing the 'best' tank. Its about unlocking options and staying away from arty lines as much as possible.

Chinese TD line to tier X.

• By researching a tier X, you also unlock the tier X heavy collector's vehicle (you're unlocking 2 tier X's by researching 1).
• There's a premium TD tank as a future purchase option you can use the same crew for.

French EBR line light to tier X

• By researching a tier X, you also unlock the tier X medium collector's vehicle (you're unlocking 2 tier X's by researching 1).
• There's a premium light (ELC EVEN 90) which is the best in the game at what it does and you can use the same crew.
• The wheeled EBR line is a tank type all on its own with a playstyle you don't get from any other line in the game.

the above unlocks all 4 tier X tank types by researching only 2 of them.

Any non-arty Russian line to at least tier 7

• By researching a tier 7, you also unlock the tier 7 arty collector's vehicle. This allows you to complete ALL country-type arty campaign missions for the Union without ever having to research Russian arty.
• I personally recommend the KV-3 as it leads to several heavy options (including the 2-barrel tanks, which are also a unique line).

UK arty to tier 9

• This allows you to complete both arty and Alliance missions that require arty. It has the second best stun per minute in the game (only beaten by the tier X French arty), which you need for both 'assist' and 'stun' missions.

the above allows you to complete every primary campaign mission in the game that requires arty by researching only one arty line (there are a few secondary bloc and coalition missions that you would need to go down to mid-tiers for though – if you really care about secondary missions). There are a couple Object 260 arty missions that are easier at tier X though, just because of larger game health pools in full tier X games provide more opportunity, but by then you're only one upgrade away.