I usually go with Merlin. Usually competitive on price. Good on options and stock. Fast turnaround and good to deal with if you cock your order up.

LBS?

Merlin. Always bargains to be had.

Another vote for Merlin.

Shop about all the usual suspects I would say, merlin, CRC, Ribble, etc... One or other will be cheapest. Other than maybe offering choices on crank length or other bits there's not much to differentiate other than the price really...

I tend to use Merlin for stuff, even if they're a bit more expensive, because they're great! You'll generally have your stuff next working day after order even on free delivery.

another vote for Merlin

It depended on the groupset. Sometimes it's cheaper to buy the parts individually.

+1 Merlin, rose bikes and bikes.de German sites can be cheap for parts too, although the euro rate is limiting the deals

Sometimes it's cheaper to buy the parts individually.

+1

I usually get mech, shifter, chain, chainset from two or three different places - but usually have one or two of the items in the spares box anyway.

If I were speccing a new bike today I'd probably get XT 11sp mech & shifter, Sram GX cassette, outgoing SLX chainset on discount, 105 11sp chain & superstar narrow-wide ring for £15.

But if starting from scratch, this is quite reasonable if you have BC discount...
http://www.chainreactioncycles.com/shimano-slx-1x11-drivetrain-groupset/rp-prod148625

Merlin are pretty unbeatable for road groupsets, but I don't find their MTB groupsets particularly competitive compared with buying components seperately from Wiggle.

Ribble are always worth checking too.