A3000 is probably best. Zarch, Conquerer, and Enter the Realm all have problems above 8mhz. Also A3000s can be fitted with ARM3 upgrades which will boost speed massively (8->25mhz usually) , but can also be slowed back to normal speed with *cache off. And there are probably a few older games whose protection only works on A3000.
Downside is double density floppies only, no built-in joystick interface (A3010 only) and no built-in IDE interface (A3020 only). Oh, and Fire & Ice won't work properly - but it's a mediocre port so you're not missing much.