I could say the Maus, A38 Valiant and/or the Ferdinand but i don’t want to re-hash does since everyone knows the flaws of those projects so here is some of the lesser know worst tank projects or ones that people don’t talk
ARL-44: France tried to built a Panther of their own with little input from other countries from possible (aside from captured German tech and stock but that’s besides the point) with predictable results. Terrible armour quality, poor maneuverability not helped by using obsolete track design and notoriously unreliable
Asad Babli: Dollar store T-72M-1, need i say more
Panzer 68: Unreliable, poorly built and poorly designed with one especially bad flaw involves the gun firing at random if the radios are used as an example, the Panzer 68 has to be completely refurbished just to fix most of the problems
OT M-60: I know it is an APC not a Tank but it needs to be mentioned here since it was one of the worst built and least reliable APCs to be ever operated, it was so bad that during the Iran-Iraq war, the Iraqi’s who operated OT M-60 explicitly told the Yugoslavs that if they want to maintain good relations with Iraq they need to cease all arms sales to them
M114: Like with the OT M-60 it’s an APC not a tank and also like the OT M-60 it was intended to replace the M113 in Reece and command roles but was rejected due to severe reliability issues and being largely less capable than ironically the vehicle that is suppose to replace
As for best tank projects i’ll list the following
EE-T1 Osorio: One of the best if not the best 3rd generation MBT to be built perfectly exemplified by the Saudi trials beating the rest of the competition in every metric which included MBT’s like the Abrams, AMX-32 & AMX-40 for example. It was rejected largely due to a lot of shady back deals done by the Americans
M60 Patton: Best American Tank to be built since the Sherman family, essentially a workhorse for many countries including the US. Is still being used by several countries to this day
Centurion: First MBT to be built and it was a very well designed and competent tank, proving the Universal Tank Concept to the world