What is a decent compressor pedal to get? I have a budget of 300 but want something simple yet effective.
$300 USD? I would go to a Keeley, but that is the only boutique compressor I've ever used, so take that with a grain of salt. Amazingly quiet, smooth, and with really responsive controls. I also thought it responded nicely to picking dynamics.
If you want boutique go with a janglebox or a Carl Martin compressor. My two recommendations.

Or if you want decent get a boss Cs-3 and get a mod kit for it for $30.00. And have a really good compressor. IMO
If you start a reply with: I have never played one but I have heard good things about it! Your opinion is invalid.
EHX Black Finger or White Finger. For that compression, where you don't realize that it is on, until it is off, the Black Finger. For that light to heavy compression, more of a compression "effect", get the White Finger.

The MXR SuperComp is also nice. I had one until I got the two mentioned from above. It is fairly inexpensive, but don't let that fool you. Very vesatile too, at least compaired to the Dyna-Comp.

I played a Keeley Comp for about 5 minutes. While very nice. I didn't spend enough time with it, to be able to recommend it.
I've got many compressors including a Kelley, a boss pedal, a couple MRX pedals and a few tube compressors that are no longer on the market.

My favorite compressor it the cmatmods dyna-comp with 3 knobs. There are actually 2 knob, 3, knob, and 4 knob versions of this pedal and I've found that the 2 knob dynacomp just didn't have the versatility that I wanted while the 4th knob on the 4 knob version didn't do much but the extra wiring for that 4th knob added some background noise. So I highly recommend the 3 knob dynacomp. the CMATMODS Ross clone is also very nice. It's got a more spongy tone that is great for blues but it also has a bit more background noise.

My all time favorite comp is the guyatone TC-X but that has been out of production for several years now. It's got a lot of background noise which means it's not good for high gain stuff, but it's tone is very very sweet.
Not taking any online orders.
For what it's worth, After trying the EHX Black Finger, MXR Dynacomp, Boss CS3.... and hearing several others online and in person

I went with the Keeley C4 (4 knob version lets you adjust clipping and attack - which is great since I use it for both acoustic and electric.)

I've never tried the Carl Martin (only heard it online), but they generally make decent pedals as well.

Gibson '57 Les Paul Reissue
Marshall TSL 601
EHX: Big Muff, Metal Muff, Small Stone, POG, 2880
Ibanez TS808
Voodoo Labs Microvibe
Analogman Chorus
Morley Bad Horsie II
Keeley Compressor (C4)
Nova Delay
MXR 10-band EQ