I'm building a 120 reef at the moment, and I am going to use a JBJ Artica 1/5hp on it because I happen to have it, and it should just barely be big enough. If I had to buy one I'd probably go with 1/4 for the job.
The best chillers are IMO Aqua Logic, but they command a high price. JBJ chillers seem to have a great reputation and although I haven't used my JBJ in a couple years, it's always been a great chiller.
Remember that the cost of chillers is incremental... it costs a lot less to go up one size now than to get one too small and have to buy a whole new one again down the road. With chillers there is a point when they get too big and cause short cycles and rapid temperature swings. 1/3 is the absolute biggest I'd use on a 120.