Really depends on your asset class. Citadel would be a safe bet for pretty much anything.
But those, Citadel, Virtu, Optiver etc... they have their own in-house software. That's not for retail use... you can't buy a lease on those.
Off the shelf, I traded with Actant, RTS (bought by Bloomberg a few years back), Sol-3 as a MM. Have a look, but I don't know if they do individual leases. Maybe Actant will, pretty sure Sol-3 won't. And I don't know the status of RTS/Bloomberg.
@CommodsTrader, what is it you're aiming to do? If you're just looking to be a retail and do a few quotes... I doubt it's going to be worth it. You're be looking at $30k /yr minimum cost for the package. And I doubt they are looking forward to providing a non-pro which will require a lot of after-sales-service...