Here is the list of top 5 exchanges, as per my experience:
1. Bittrex
2. Kraken
3. Livecoin
4. Bitstamp
5. Poloniex
I would have listed Cryptopia and Wex as well, but they are not doing very good right now.
Wow, I thought that no one is using Kraken now.
Hmm.. I used to do most of my XLM/BTC trading in Kraken. The staff is quite supportive as well. Once my XLM was stuck after I forgot to add the memo, but the support guys resolved the issue amicably (although it took some time).