I would say XNA - it's C# based but very easy to learn and extremely powerful, and has the backing of Microsoft which means it will probably last well and be updated often.
Yes, it's not cross-platform, if that's an issue for anyone, and as Andy already said, it does have some pretty fearsome requirements, which would be a problem for a casual 2d game, but perhaps not for a mainstream 3d game. I would certainly agree that, if you're comfortable with Blitz3D syntax and particularly if you're comfortable with BlitzMax, C# will not be much of a leap.