The N1MM and MMTTY is very popular, as is the 756 Pro series, N1MM is a super contesting logger, supports just about everything, you can also run SO2R if you have the hardware. Almost an unfair advantage :-)
I run the Pro II with a home brew FSK interface that can be found here http://www.aa5au.com/rttyinterface.html
Don's little 2 transistor deal works fantastic on FSK, I have to disagree with Tigertronics comments that AFSK is better than FSK, I have used both and there is no contest as far as which is better, also as the previous poster mentioned, with the ACC connector it's very easy to interface the 756 radios for RTTY, for about 5 bucks you can have an interface that rivals a 150 dollar Rigblaster Plus on FSK, add one wire with a couple of resistors and an isolation tranformer robbed off an old modem from the mod input on the acc to the sound card and you have an all modes as well as FSK and no worries of adjusting the mic gain between modes, etc, it's all in thru that ACC connector
Read Don AA5AU's review of the 756Pro III on his site, I think the Pro series is one of the better RTTY radios on the market, it has a lot of features especially for RTTY, like a built in rtty decoder (memory transmit on the P-III) and the twin peak RTTY filter, I went from a SDR-1000 to the Pro II and I love this radio better than anything I else I have owned, and it's as good on the other modes as it is on RTTY, you can get a Pro II for 1400-1500 and a used P-III for just over 2K.
One more thing, if you go with a 756 rig for contesting, you must have a PC interface, don't waste your money on an Icom CT-17 interface, there are many designs out there for Icom interfaces for very low cost, I recently built a direct copy of the CT-17 by ordering it part for part from Mouser, the parts cost including shipping was about 13 dollars!! I think the last price I saw on a CT-17 was well over 100 bucks!
I read somewhere that RTTY was the fastest growing mode in ham radio, you work a couple of contests and you will see why, it's a lot of fun and has been my most used more for the last 3 or 4 years, lotsa DX too!