LOL okay that's fine but I meant the question more as what load are you specifically going to drive with the amp?
Because that makes a big difference in what people are going to suggest and what options you have.
If your sub is going to be 4-ohms total then a 300W into 4 ohms mono amp is going to be more expensive as you'll proabably have to get an amp that does more power into a lower impedance and therefore justifies the higher cost. But doing that you'd be "wasting" power (and money) running the amp at 4 ohms when it's designed for maximum power at 2 or 1 ohm.
There's also other amps that put the same power out at any impedance as they have advanced regulation systems incorporated; however note these are also typically more expensive. IMO these types of amps are a general waste, though that's just my opinion.
Or another thing you might do (if you're planning on a 4 ohm load) you might want to get a 2ch class A/B amp and bridge it to drive the sub. That would work out well and you'd be getting the maximum power out of that amp.
Also what impedance you drive changes the selection you have as well. If you wanted to drive a 2-ohm load there are quite a few mono/class-D amps out there that are well suited to putting their maximum power into a 2-ohm load. Most mono amps from companies like Kicker, MTX, Infinity, and others are all designed for optimal performace at 2-ohms.
So as you can see it's a good idea to first decide what load you'll be driving specifically, before asking "what 300W amp" to buy.