I play on PokerStars myself so I feel you here. It's the best there is. Now, I never played mid- or high-stakes so I don't know what's there, but I can give you some insight based on my experiences.
I don't think PokerStars needs any more introduction. They have the largest player pool, the biggest variety of games and you can find games non-stop.
Full Tilt Poker Are owned now by PokerStars so the possibility of another Black Friday to return on this site is slim to none.
- You can transfer money from FT to PS and vice-versa quickly and easily. Very useful if you find a nice promotion on the other site;
- The games are somewhat softer than those before Black Friday and definitely softer than PokerStars;
- Nice, decent traffic outside peak hours;
- Best software in graphics section, if you ask me
- Low traffic at Rush (for those who dig Rush);
- Even if the graphics is awesome, sometimes it can be too much for grinding. In this respect, PokerStars rocks!
- Low traffic at higher stakes in non-peak hours.
888 poker can be a very good choice if you want a quick buck and very fishy plays. So fishy that, coming from PokerStars background, you'll either won't believe it and/or start laughing. I surely did.
- One of the fishiest site I played at. At stakes up to NL20 there are easy money. Just play ABC and watch for things at the other players that you can exploit. And you will not wait too much;
- Tournaments are fishy too, especially at low buy-ins. If you pay attention, you'll end up ITM more often than you should.
- Traffic. You'll find decent traffic all day long only at NL2 and NL10. At the other stakes you will be forced to play in the evening or night, because at other times there won't be any tables open. Even so, expect 8-9 tables opened, in the best case, even at peak hours;
- VIP Scheme. The 888 VIP scheme has six tiers. The lower 3 tiers can be easily achieved but they offer ridiculously low rakeback (2% or so). The higher 3 tiers offer better (but hot great) rakeback, but it's very hard to reach them because of the low traffic described at 1. I can say that the money earned at the tables makes up for the low rakeback, but money earned and rakeback are two different things;
- The shop. It's rich in offers, but the points are, again, hard to gather.
About Titan Poker I have mixed feelings. I played a few years go and the games were so nitty...
- Nitty games. Can be beaten straightforwardly
- Graphics. It's a personal preference, but I don't like it at all. Plus there are some annoying sounds, which can turn into a nag house if multitabling;
- Promotions. Just a few...;
- Point conversion. You can have only cashback (there is no store).
I didn't play on the other sites you mention.
Hope it helps.