Things start getting complicated when trying to figure out what Tier 5 tank you'd like to research, especially based upon how much XP and silver it's going to cost! In general you're looking at less than 50,000 XP; the British tree is slightly cheaper, German tree has pretty good armor.. although with some of the newer nations, there's a really good mix. It's a good idea to try out a variety of nations and tank types before really starting in for some of the higher trees. There's nothing worse than getting to Tier 9, and realizing you really don't like the Tier 9 tank! LOL, happened to me.
My personal recommendations for tanks:
Light tanks: US Tier 5 Chaffee. It's a fun tank, very fast, and the previous tanks are a lot of fun to use. It costs a LOT of XP, but really shouldn't even be a Tier 5 tank but higher. Otherwise, the German Tier 5 Leopard is fantastic...
Medium tanks: US Tier 5 M4 is great, I absolutely loved the derp gun package(105 mm, huge damage, often one hit kill, shots arch instead of straight line).
Heavy tanks: Soviet Tier 5 KV-1 packs a major punch and has some great armor.
Tank Destroyers: German Tier 5 Pz. Sfl. IVc--if you get the first shot especially from hiding, you'll probably get the kill. Great tank, just don't get flanked or hit.. if you think you're going to get hit, then get the British Tier 5 AT 2. That tank is ridiculous, as long as you don't get tracked and shot from behind. It has amazing armor for it's class.
Artillery: To be honest, I really hated using artillery. If you're going for one though, most of them are half-decent. I liked the German Grille, the US M41, and the Soviet SU-122A were all good.