Which bundle site is the best?

There is humble and then everyone else

Personally, I prefer Humble Bundle or Indiegala.

Bundlestars

Groupees.

Groupees. HB is ok but groupees always has the good stuff.

Actually every bundle site is pretty good, it depends on what you're looking for... I couldn't go for a "best", some sites are on their first bundles, some (like HumbleBundle) are already quite popular... You'd have to define "best" for I could answer ("best" based on what ? popularity ? bundle frequency ? number of games ? price ?)

indie gala group buys have the best price per game, groupees has the best mix of quality + cheapness, humble with the best quality

Indie Gala.

actual Groupees

Groupees and Bundlestars. If you ask this last year, I would have said Humble bundle.....but quality of their bundles drastically went down this year. I think only 1 bundle or 2 of theirs was at least decent compare to 5+ during 2nd half of last year.

If you want the best deals (money/game value) then probably Groupees. They are rather into newer indie games, but they give away quite a lot "oldie but goodie" games. Plus their bonus system is great, if you want cheap content.
The best names are usually from HumbleBundle. They are quite okay in my opinion - they start to peak out of the 1$ a heap of games, BTA for a few more - system by setting fix tiers, but I think it's sometimes understandable from developers side, if they don't want their 20$ game made into a 1-3$ "tier". (If BTA is 3$, then the games in it actually cost only additional 2, as you pay 1$ for tier 1)
The best thematized bundles are Bundlestars':I don't like all bundles, because for example, 3 simulations bundle in past half year, but they are quite decent, no really weak or "oh my good what IS this?" bundles.
Blink bundle varies a lot. I'd say the are between Bundlestars and indiegala. Sometimes great, sometimes generic, casual, 100th time bundled games.
I'd say Indiegala is the worst by far. They have usually cheap bundles, but almost all games got mostly negative criticism from players, or contains "generic indie games": like another match-3 game with no originality, only on greenlight. It's a rare to see any "that's great!" or "wooow, dem retro feels, I want it" games.
So in the end in my (roughly objective) opinion: Bundlestars (great bundles, but maybe not aimed for you) > Humblebundle (best titles - but not dirt cheap) = Groupees (lots of good and varied content, for really cheap)> Blink (varies) > Indiegala

I'm always with groupees since I like having music/e-books in bundles as well as games. Also they usually offer lots of quality content for fair price (unlike HiB nowadays).

Groupees

I used to like humble bundle, when I could pay 0.01$ and get some DRM free games. I would pay 1$ if I wanted the games on steam. Now they all steam-only games and the only decents are the beat the average games.
And once per (half year?) u have a bundle with a decent game. All the rest use to be craps no one care about