Many are saying that Bradleys and Marders would be sufficient. No they would not.
Bradleys and Marders do a great job in protecting the mech infantry supporting the MBT's and taking out IFV/APC/etc, Bradley has the TOW to take out MBT's, but it only carries a few and it is not designed to go against fortified positions.
The MBT's are needed to take out fortified positions and red MBT's etc, to make a breakthru possible. It really is a matter of combined arms (so they need the arty and MLR's amomgst much more).
There is really nothing that could provide the same combination of protection, firepower and mobility that MBT's bring with them.
There just isn't enough challys unless UK ships them all. The Abrams are way more complicated than Leos to bring fast enough to the battle field.
Leos: Europeans have the vehicles, the spares, the factories and the trained professional support throughout Europe. As an example the Poles could repair Ukrainian Leos already without further training.
Leo is the obvious choice and could be provided very fast.
The estimate is that an Ukrainian, experienced T72 crew could be converted as combat ready Leo crew in 6-8 weeks, maybe even faster.
Finland and Poland has experience in going from T72 to Leo2, that experience should be used in training.
Ukraine is consuming their inventory faster than it is being replenished with soviet kit as the west is running out of soviet vehicles. The western MBT's are needed in quantities, WITH other western weaponry, be it vehicles, armour, munitions, flying machines (UAV, rotary, jets...)...
The sooner Russkies are fought out of Ukraine, the less different new weapons systems need to be delivered (and by that: what will be delivered once UA runs out of Migs and Mils? Are we ready to replenish them with Vipers or something to its kin?)