First off with straight talk you have to decide if you want to use the T-mobile network or the AT&T network because they offer service on both in most locations. It depends on what city you are in and who has the best coverage. However when I brought my Nexus 4 to straight talk the only option was T-mobile but that was fine because I am in Atlanta and T-mobile has very fast HSPA+ speeds in Atlanta which is what the Nexus 4 supports (No LTE on the Nexus 4).
As for phones to bring to straight talk pretty much any phone that works on tmobile or at&t should work in theory. If you are looking to buy new this is what I would recommend:
1. The Nexus 5 ($349 purchase directly from Google) - Excellent phone with LTE support. dont need to say much more. there is plenty of info on the Nexus 5 out there on the internet.
2. The just announced Huawei Mate 2 ($299 purchase directly from Huawei) - This phone is unlocked and off contract just like the Nexus 5 and comes with a SIM card for tmobile or at&T (although to get it to work on straight talk you will still probably have to buy one of there SIM cards). It has a 6.1" screen (great if you are into BIG phones, and perhaps most impressive a massive 3900 mAH battery which should provide some of the best battery life out there of any phone period).
3. Moto X - purchase the unlocked no-contract GSM version for use on straight talk. Motorola and other outlets are constantly offering deals on this phone so it can be had for anywhere from $299 - $379.
4. Finally if you want something cheaper than $300 look for a used Nexus 4 on ebay, amazon, etc. It's a great phone and because its a Nexus you will always get the latest OTA Android upgrades quickly. It can probably be had on Amazon or elsewhere for under $250 depending on the condition of course as it will be used.